since it's a new build go with a quad core. a lot of new games that are coming out are going to be optimized for quad cores so, if you want to futureproof your rig get a phenom. i recommend the phenom 9750 i have it and the performance is sick!
what about the 9950 its black edition but it pulls 140W so get a good mobo. the 9950 i think can overclock to 2.8ghz or 3 from it's 2.6. it's amd's king of the hill processor. 2.6ghz unlocked multiplier ( blakcedition).
I would recommend the phenom cause even i think the x2 is faster in games. the phenom is more future proof because more and more games are gonna take advantage of four cores rather than two. also you can should be able to decently oc the cpu.