The vast majority of games don't benefit at all from dual -> quad cores. An exception would be Supreme Commander, but even then, a higher clocked dual core can easily make up the difference:
http://images.tomshardware.com/2008/...rs/supreme.png
Supreme Commander is very CPU limited and performance scales almost linearly with clockspeed. Extrapolating from the Phenom 9600 results, a Phenom 9100 would score about ~36fps, putting it below an X2 6000+, even in a heavily multithreaded game.
At the projected $100 - $130 pricerange, you're looking at C2D E4600 or X2 6000+ as alternatives. I would say both these chips will outperform a Phenom 9100 in gaming performance.
Let's not kid ourselves - by the time most games actually start taking advantage of quad cores, a Phenom 9100 would be obsolete. Even upon release, it'll be BY FAR the slowest QC CPU available today, let alone in 2 years time when we'll be running 4GHz+ octocore Nehalems and Bulldozers. It'll be a bit like running DX10 games on a 8600GT - yeah, it'll run with all the fancy features on, but at 10fps. :rolleyes: