Athlon II X4, especially since the Core 2 Quad is at the end of it's life. You're not going to be seeing any new models for LGA 775 so it doesn't make sense to invest in that platform for a new build. True Core2s have better clock per clock performance but the higher speed of the Athlon II more than make up for it and it's cheaper.
Invest in a good AM3 board so you have more upgrade options latter on. The new Bulldozer AMD CPUs probably aren't going to work in AM2/AM2+ boards after all.
Have you thought about a Core i3? 2 cores but 4 threads and the overall cost of the platform is only slightly greater then the AM3 platform. However you'd have the option of dropping in a far more robust cpu later on if you spend good money on a higher end mobo.
If that's not an option I'd consider a 955. It's $60 more but the rest of your platform stays them same. And the performance of a 955 is on par or better then the old Core 2 Quad Q9550. Add to that it's a Black Edition, it's simply the best budget CPU around.
Yeah I've used a few 955s and they go toe to toe with my q9550 easily. If you have the budget go for it. The phenom 2 is better than the athlon 2 x4 for gaming but they are both really good for their price points.
It is the much better deal. In fact, the Athlon IIx4s are one of the best performance/$ processors on the market today. I got my 2.6GHz Athlon IIx4 for the same price as this 3GHz model is listed, and it was still one of the best performance/$ CPUs.
For gaming it works well but we really need to know what the rest of your system is to best help you.
So like you said though, the Athlon IIx4 is cheaper and more powerful (if only by a small amount).