any single GPU that you can afford, i had a Q6600 (at 4.05ghz) not a long time ago and it was running great with a 680 in about 10% slower difference over the 3770k (3.5ghz) performance with the same card
This thing will still handle any single gpu you'll throw at it... The only big factor is multithreading as newer cpu are more optimised on that point so in games like BF3 you maybe see some stutter when it load or save...