Uh, Mi1ez, he's already got a quad core and is asking if turning some cores off will make games run better.
@xtian63 - no, turning off cores will not make any game run faster. Unless heat is your limiting factor on your CPU overclock, the chance of turning off a core improving your overclock is virtually nil. Nearly every modern game needs a dual core at least, and in a recent Toms test, most benefitted from a third core too. If you're running your quad with all four cores active then the game can use 2/3 and that leaves 1/2 to do background stuff so your FPS won't be hurt when your anti-virus or Windows decides it needs to do something.
If CPU temperature is your limiting factor on your overclock, then possibly disabling a core might gain you a slightly higher stable overclock, but only trying it on your system will tell you if that extra few MHz is worth the loss of a core on that particular game. My money would be on "no", but each system overclocks differently and each game requires different things from a CPU, so the only way to test this is to try it.
All that said, for 99% of games on 99% of systems the limiting factor is the graphics card - are you running dual 5970's or something similar? If not, your CPU will not be limiting you outside of (possibly) FSX.
My advice - leave all 4 cores active, overclock them as much as you can if CPU power is holding you back, and enjoy your system!