Since the price of the Q6600 dropped, I'd aim for a quad core.
Granted, games at the moment are not optimized for multi threading but it's definitely coming in a near future. The performance gain might not be as one would expect in games, but if you're building a multi task computer (homework, video, games, etc.), than it's something to consider in my opinion. Plus, you'll probably want your computer to last a couple of years, and if that's the case, I'd aim for a quad core. (The Q6600 is selling at ~260$)
But hey, both Intel and AMD will be releasing by the end of the year their newer models, therefore you can either wait or buy one now. Moreover, you can expect something from Nvidia by ~December.
For those interested, Intel has release a prototype of a 80 cores, and pledges to release one in five years.
Here's the article:
http://news.com.com/2100-1006_3-6119618.html