It depends heavily on the games you're talking about. Bad Company 2 takes advantage of Quad Core processors, for example. So, having a Quad Core for that game is going to be better.
We don't know whether or not Crysis 2 will support Quad+ Cores or HyperThreading, so that something that we'll just have to wait and see.
I personally prefer a fairly decent Quad Core over a Dual Core, just to be future proof. I have the Q9400.
Anyway, it just comes down to what your budget is, what games do you have and/or plan to get.
Iceman