i5-2500/2500K is $220 and is the best choice for gaming. If you want to spend another $100 you can get the i7-2600/2600K, which has a little more cache, a slightly higher clock speed and hyperthreading.
Both cpu's (K version) will overclock to 4.5-4.8ghz. The K models are the unlocked versions.
Any SLI certified P67 motherboard would work and some 1.5v DDR3-1600 dual channel memory.