A fast single core would be better for TF2. I've got a quad that runs 2.5 stock and it was bottlenecking. TF2 seems to want 3 or more, boosted it up to 3.3 and it runs fine.
Get yourself a good system monitor. Speedfan will cut it, but I like everest. If you're playing at 90% or more on a single core while all the other cores are near idle, that game isn't optimized for multiple cores.
Off the top of my head though? Age of Conan is.