Ill be honest I have seen alot of people try to build off of old systems and end up dumping more in than they needed to. You are best off to start fresh in my opinion. I just built a new gaming rig for about $650 dollars and most people would consider that on the low end. Thats my opinion and WR2 helped me with some questions so he might have some guru insight that im overlooking here to save one of those but I cant feasibly see it.
System specs:
MOBO: MSI P67A-C43
CPU: 2nd Gen Intel I5 3.3ghz
RAM: One 4GB stick of DDR 1333 Pioneer ram
Hard Drive: Seagate baracuda low power 5200rpm 1tb hd
Opticle drive: Generic dvd r+w
PSU: Ultra 550 lifetime series
Graphics Card: MSI N460GTX Twin FROZR II
This runs most new games on med-high if you want to check the specs.
Best of luck LOGIC.