Im not sure that you can buy an already build Desktop for gaming here but you can check at sofitel.

Here is the least specification that you might need in buying one.

Processor at least: i3 or i5 > High End:i7 or Quadcore
Memory at Least: 4 Gig DDR3> 8 Gig DDR3
Motherboard Asus ROG or Gigabyte
Hardisk 500GIG sata 7200Rpm
Video Card > Nvidia 360 series Nvidia 485 series
Monitor > High resolution Should be
Windows 7 Ultimate

And Very Good Colling Kit! XD

check our nvidia.com for the latest video cards.

Good Luck