Building a new PC is easier then ever, selecting the correct components however is not. It takes alot of knowledge to know what is good and what is not. Many aspects of the PC market change so rapidily that what may be top end today may not be in a month. One of the primary mistake's people make when building a machine is they assume Ghz means everything and that all hardware is effectively the same. This is FALSE, Hardware is not equal, GHZ is meaningless and there is a black and white difference between Prefab or even configure to order machines and an Actual custom machine.
In your case you desire to play FFXI, however I would assume that you will play some games advancing down the road so while a Top end unit is not the solution for you, you already have a fairly nice budget to work with. For this reference I am only going to use Newegg.com so I point everything out from one place, as a reference many of the parts here I point out can possibly be found cheaper through different vendors using pricewatch.com, however I created this from one simply for ease of reference. As far as assembly is concerned, if you question your ability or feel overwhelmed, simply have a knowledgable friend or even a local tech shop assemble for you, they will usually do so and install windows for a nonimal fee.
So with that said, here is the hardware I have picked for you and the reasoning why,
First and foremost is the Case, There are alot of choices on this however in your case I am attempting to save money while still providing a quality case, and since you seem to move a bit I gave portability a consideration as well. Also in this instance Motherboard, power supply and cooling played an important factor. Most cases will simply be the case and psu, but in the case of Shuttle since they produce an excellent motherboard Its a hard value to beat for the overall package. Even high end gamers such as myself who overclock and demand the absolute best performance are hard pressed to knock this unit, As well this case also utilizes the ICE cooling which is a Heatpipe based cooling solution for the CPU, and due to its configuration it puts Zero stress on the bracket or motherboard and is simply put one of the best cooling systems available outside Watercooling and above.
http://www.newegg.com/OldVersion/app/Vi ... 471&depa=0 $289 This is a socket 939, now my initial inclination was to go socket 754 on this and save a bit of money, however since 754 is being phased out and since you have a bit more room then the last budget pc I built, I figured you would benefit from the added upgradabilty of this. So if you decide to up your cpu to a dual core at a later date you can simply upgrade your bios and keep your case/mb and everything.
Next the obvious CPU, based on the case/mb you can pretty well guess its going to be an AMD. For reference I am neither Pro AMD or Intel, I go with whichever cpu offers the best bang for the buck. Right now especially for gaming but really for anything the Athlon 64 is undisputed, Intel is simply not competiting. The intel 64 bit was not built as a 64 bit chip and it does not function well as one, it performs no faster then its 32bit counterpart unlike the A64 where there is a noticable difference. Currently there is no reason to buy an intel chip, that may change soon its hard to say, Intel is being tight lipped about it.
http://www.newegg.com/OldVersion/app/Vi ... 502&depa=0 $195 Athlon 64 3200+, this is fast enough to run FFXI without the slightest struggle and fast enough to carry you at least 1 year or two into future games. And as I said the motherboard is fully upgradable to the Dual cores since this socket will be around a while.
On to the next topic, RAM. There is no way for me to convey to you how important ram is without going into a long diatrabe of which most you wont understand. Lets suffice to say people are simply badly misinformed with the importance of ram, quantity is important yes, however quality is more so. I can easily prove to you that 512mb of Premium memory will easily outperform 1gb of generic, it comes down to how fast the Ram actually works, not how fast it Claims to work. But as I said, Very very long topic. Fortunatly for those in the know buying premium memory is not as arduous as one would expect nor as expensive.
http://www.newegg.com/OldVersion/app/vi ... 440&DEPA=0 $81.99 for 1gb of Corsair Value select. Corsair is hands down the best Ram on the market right now and has proven it time and again. Talk to any overclocker and 90% of the time its what they use. Now while the High end gamers prefer the XMS series unless you are overclocking there is no need to go past the Value select. It carries the same Lifetime warranty that the XMS does at half the cost. Yes thats right I said Lifetime warranty, find that on generic.
Hard Drive, Alot of choice here some prefer Maxtor, some prefer Western Digital, some prefer Seagate. I went with the best deal on the website which happens to be a western digital.
http://www.newegg.com/OldVersion/app/vi ... 154&DEPA=0 $114 250gb SATA hard drive, Lets simply say you can download alot of stuff on it. Also it is the SATA intereface which is faster then the older standard, plus uses a smaller cable which allows for better airflow in the case.
Optical drive, You know these are an amusing thing as basic CD roms are still $25 and DVD roms are $36 retail. Why on earth buy a basic when you can buy a burner for a scant amount more. This is a lite on, and while you may not be familiar with their name, suffice to say they sell their cores to the majority of OEM computers, so to say they are a pretty good burner is an understatement. Also a
$44.99 This is a Steal for a 16x dual layer DVDRW/CDRW, it handles all formats.
http://www.newegg.com/OldVersion/app/Vi ... 966&depa=0
Video card, This is one of the most important parts of the unit. The biggest mistake most people make is they go for a budget card thinking to save a bit of money. In the end most spend more because the card lacks so many features they end up upgrading far more often then someone who buys a top end card. However you do not have to buy bleeding edge but you want a good value for your money. Right now based on your budget this is the best bang for you.
http://www.newegg.com/OldVersion/app/vi ... ory=BROWSE $295 A couple of Important notes for you, #1 LIFETIME warranty, yes there is that lifetime mention again, I have dealth with PNY and I assure you its a decision you will not regret. You should get no less then 3 to 4 years of solid gameplay out of this card, right now its one of the best performing cards for the money. Coupled with the fact that if it dies in say 3 years PNY will replace it most likely with a then current generation card. If you get 4 years out of it like I did my last PNY card then you cant really complain about the amount of money because you will never get 4 years of excellent gameplay performance out of a budget card..
Sound card...You can buy one..but you honestly do not need it. The Integrated sound in the Shuttle case is Superb.
Windows, your choice, I selected pro as I particularly dislike Home and at $149 its hard to argue, but if you want to save the money home edition is $88.
http://www.newegg.com/OldVersion/app/vi ... ory=BROWSE
Now with that said that leaves monitor, you have not mentioned anything as I typed this so I will guess.
http://www.newegg.com/OldVersion/app/vi ... 171&depa=1 $255 only two things matter in LCD monitors, contrast Ratio and Response time. This monitor has the best of both and at $255 is an incredible value. It is a gaming monitor, it will not ghost your games it will look good playing them and you wont regret it. You can of course go larger it depends on what you want to spend.
So I am going to attempt to link the shopping cart so you can see but your total is as follows in case you cannot.
$1396.93 minus shipping, there was a instant rebate on the case, I do not know where you live so I chose the most expensive place to ship which is CA, due to free shipping on alot of stuff its
$30.79 for Everything which is damn cheap. Also Newegg is pretty much second day.
GRAND TOTAL $1,427.72, Feel free to shop around but you will be hard pressed to find Any Pre fab machine that even comes close to competing with this for overall value. Also as a note everything that does not have a lifetime warranty has a 3 years warranty, all Prefabs are 1 year and you have to pay for extended.
Shopping cart if it links
http://secure.newegg.com/Oldversion/app ... otionCode=
Oh yes, on keyboard and mouse..pick one you like its all about personal taste they effectively all work the same.
Though now I've gotten more high end parts so I finally got a 500watt, still the same cheap case though. 
