I HIGHLY recommend building your own system. This months maximumPC magazine and/or online articiles will guide you through all of the steps.
It is surprisingly easy so don't be intimidated. Seriously. You will get a lot more computing power per $ than purchasing a prebuild system. You also have the internets to be able to ask questions. I certainly dont mind helping out at all.
The specs i recommend are:
1. Core i7 920 - this is easily overclockable for higher througput but it isn't necessary. Microcenter has been running specials on it for $200 (plus tax because it has to be purchased in store). This processor is amazing and my sim speed stays between 9-10 on 5v5 maps. (i was able to get it to drop to 8 when i was burning a dvd and listening to music in the background)
2. Motherboard - I recommend the following brands: Asus, Evga & MSI. I have always had excellent experiences with all three however i typically fo with Asus in my personal computers. If i had to pick one for you it would be the asus P6T Deluxe but you could aslo go with the P6T SE LGA 1366 $250 (if you have the money then the Evga Classified looks awesome and the Asus Rampage II is awesome but those boards are for overclocking.)
3. Ram - 6 gigabytes is the sweet spot. (three stix of 2 gigs) DDR3 PC1600 with timings at 8-8-8-24 or less (lower numbers are better). there are a few great brands (OCZ, patriot, crucial, Corsair etc.). Personally i prefer corsair. Ocz has some at newegg that is $115 after the mail in rebate. model: OCZ3G1600LV6GK
4. DVD burner - $30, sure get a cheap one, why not? newegg has a samsung for less than $30 model: N82E16827151188
5. Power supply - i would go between 650 - 750 watts. PC Power & Cooling arguable makes the best. Other good brands include Corsair, Thermaltake, Sparkle etc. I've had a Lot of Antecs burn-out on me so i dont use them anymore. Newegg has a 750 watt corsair model for $100 after mail in rebate. model: CMPSU-750TX
6. Case: There are a lot of good case manufacturers (antec, thermaltake, lian li, silverstone, Zalman and many more) just get what you would like. $150
7. CPU cooler, you can use the stock cooler, it's good enough. You could also get a thermalright ultra or zalman cooler $80 plus thermalpaste (arctic silver) $10 or so.
8. OS: Windows 7 release candidate. This will be good until March when a full version will have to be installed.
Total spent thus far: $935 budget left = 565
Now that leaves us with the last 2 components - video card and hard drive.
Do you want a big hard drive? I will assume so. If you play a lot of games then i would dump the rest of the $ into the video card, if you do a lot of video editing then i would recommend velociraptor hard drives along with a storage drive.
(assuming SSDs are out of the budget range)
large: 1.5 terabyte seagate: $110 model: ST31500541AS
Use the rest of the money for your video card:
I typically go with nvidia but ATI's are good too.
GTX 275: $220
GTX 285: $260 after MIR model: 01G-P3-1182-AR (evga)
ATI 4870 X2: $370 xfx model: N82E16814150349
Mouse+keyboard is yoru preference of course. you would have about $100 to use. (or save and get windows 7)
Im also assuming you have a monitor that you use.
Use these specs to compare against whatever pre-builts you are looking at.
I hope that helps!