OMG of course you disable cores. Hell, I've done 1 core 1 thread, 1 core 2 threads, 2 cores 2 threads, 2 cores 4 threads, 3cores 3 threads, 3 cores 6 threads, 4 cores 4 threads, and 4 cores 8 threads on my old I7 860. It can do any configuration.
As for gaming, I would go with the I5 750 honestly. Hyper threading does nothing for gaming, unless you only have a dual core like an I3
Just a Noob, plenty of P55 boards are cheap. My personal favorite is the ASRock P55 pro for $110. You can only use a single x16 slot with that though, you have to buy the P55 extreme for 8x/8x SLI/Xfire which is $140...except of course right now there's an open box extreme for $80 on the egg. A steal of a deal with an I5 750 and 4gb of G.Skill ddr3-2000 cas 9 memory would really kick ass for gaming. I personally like just running one strong card instead of SLI or Xfire, so have full 16x/16x is pointless. There's no game at any resolution a GTX480 or a 5970 can't play.
The Phenom II X6 is just that, a lame Phenom II with 2 extra crappy deneb cores stuffed on the same die. There's only a hand full of games optimized for quad cores, it won't be for most likely 1-2 years before games will utilize more than 4 cores and even in the beginning a super strong quad like the I5 won't bottleneck you if you overclock it. I ran mine at 4.0ghz 24/7, a beast of a gaming CPU.