How often do you update your rig ?
If the answer is less than every year, get a 4 core processor. Be it an i7 (well, I'd wait to see what the future i5 can deliver) or an AMD one : atm games don't use 4 cores but it should be the case soon enough to make dual core obsolete.

[Linked Image from w3.the-kgb.com][Linked Image from w3.the-kgb.com]