Pick a C2D any C2D and you will be fine. You only come close to limiting yourself is you go with an old single core CPU.
How much percentage difference between E4400 compare to E6420 pairing with 8800GTS 320Mb? What about E4300 and E6320? It is huge? Or just merely 5% or something? Does it have any impact loss of FPS when I do it on gaming? Thx.