Build your own PC, it will cost much cheaper and it will be more reliable.
How much is your budget exactly ? and do you want to buy just the case ? or would you prefer a monitor, UPS and peripherals? Also, at what resolution are you planning to game on ?
A user called Avolio just brought a rig that I'm going to build for him on Thursday. These are the specs :
Gigabyte Pro Odin 800W PSU
Intel Core 3.4GHz - i5-3570 6M/LGA1155 Original Boxed
Gigabyte GA-Z77X-D3H (rev. 1.0) (DDR3) (LGA1155)
Kingston HyperX Blu 8GB 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory Model KHX1600C10D3B1/8G
2X WD 1TB WD1002FAEX 7200RPM SATAII 64MB Black Edition (RAID 0)
Gigabyte GV-N670OC-2GD 1.0 GeForce GTX 670 2GB DDR5
LG GH24NS90-24X DVDRW - Dual Layer SATA Black OEM
Cooler Master Mega Flow 200 R4-LUS-07AB-GP 20cm Red LED Fan
Cooler Master Hyper 212 R4-L2R-20CK-GP 12cm Red LED Fan
ThermalTake Chaser MK-I case.
I think it cost him around 1200 - 1700$
All parts are from PCandParts.com. They ship the parts to you and you pay on arrival. They do build it for you if you want, but I would really prefer if you build it yourself.
Here is the link of his topic