Only benchmark I can find for starcraft 2 with an E8400 in it. http://www.pcgameshardware.com/aid,705392/Starcraft-2-Beta-CPU-benchmarks-x-Core-i5/i7-leading/Practice/
Phenom II is roughly AMD's equivalent to Core2, you won't see much of a boost unless it's a game that can utilize 4 cores, than a Phenom II X4 is better than a Core2Duo. A Phenom II X2 would be roughly equal to the E8400 in gaming.
Starcraft 2 only utilizes 2 threads by the way, extra cores won't help unless you've got other programs running in the background.