Newegg.com
If you can't build a PC, that's the best place for a cheap, decent gaming rig. If you CAN build, buy your parts from there and build it together, you'll usually save between $100-200.
3 important things for a gaming PC.
Proccessor (CPU)
Graphics Card (GPU)
RAM (Memory)
But you'll also need a good power supply to run it all, 800W is pretty decent.
Intel i7 is one of the best CPUs.
GPUs- you can either go Radeon or nVidia. Both are pretty nice, I myself use Radeon. If you play only those two games, then a Radeon 5770/nVidia GTX260 is plenty good enough to max it. If you play more demanding games, then a Radeon 69xx/GTX460 should be able to run pretty much every game out there.
4GB of RAM is all you need, anything over 6GB is overkill. I know guys with 12GB and it scares me.
If you play a lot of games/make videos/take pictures, then a hard drive is 1TB is plenty of space, and you can always add more if needed, so no big deal here.
Sound cards - an onboard card is fine IMO, usually adding a sound card adds more trouble with drivers and conflicting programs, at least for me anyway.
Motherboard - you should try to get one with at least 2x PCI-E slots for any future upgrades.
Windows 7 Home 64bit is good enough, no need to buy that proffessional stuff.