What's your budget Razor? What is the MOST you will spend on building a computer?
As the guys above said, custom made is the way to go. You won't have as many compatibility problems and you will be saving a lot of money by not buying a prebuilt comp.
here are a few suggestions for you:
Memory: ($225) CORSAIR DOMINATOR 6GB (3 x 2GB) 240-Pin DDR3 1600 (PC3 12800)
CPU: ($280) Intel Core i7 920 Bloomfield 2.66GHz LGA 1366 130W Quad-Core Processor
[Don't get the i7 that are better than the 920, if you get watercooling you can OC the 920 to over 4ghz easily and save hundreds]
GPU: ($195ea) XFX HD-489A-ZDFC Radeon HD 4890 1GB 256-bit GDDR5 PCI Express 2.0 x16
(I would recommend getting two of these if you can afford it, or something similar to this quality if you prefer nVidia)
Any motherboard that will meet the requirements of the products listed above will work, but I recommend getting something of high quality. Also, any Full Tower ATX case will work but again, spending more on a GOOD case is never frowned upon. If you get a mid tower you might have problems getting the new GPU's to fit.
PSU: ($320) SILVERSTONE ST1200 1200W modular w/ Active PFC (80+ efficiency certified)
[I only recommend getting this 1200w psu if you plan on running 2 or more GPU's in the future, an 700-800w would probably be fine for just one GPU]
The build I suggest (above) will run you about $1500 if you keep your old hard drives and optical bays, but you will be close to the front edge of technology.
If you have money to blow here are a few suggestions for hard drives:
Get an Areca raid controller, I think the cheapest one I've seen runs close to $400. These are meant for high end server setups but you will need something of this quality for the hard drives listed below as Marvell still hasn't perfected their raid controllers and their Serial Attached SCSI (SAS) tends to crap out.
Get some Fujitsu 150gb 15,000rpm SAS hard drives (they are server hard drives and are BUILT TO LAST). They blow the WD Raptors out of the water when it comes to speed and stability. These will run you close to $350/ea and you will need the Areca controller card or a motherboard that supports SAS (motherboards that support SAS have Marvell chips and tend to fail often)
If you have the Areca controller you will be able to set up raid using:
RAID 0, 1, 10(1E), 3, 5, 6, 30, 50, 60, Single Disk or JBOD
I am only suggesting getting an Areca for it's strong support of SAS.
Get a couple of the fujitsu's and put it em on RAID0 and you will have super fast hard drives. Server hard drives are only recently becoming popular with the gaming community, but SAS has been ahead of SATA for years now.
Just some thoughts