The x4 Athlon II at 3GHz is, on average, faster than the Q8400 on average. Here is a 2.9GHz Athlon IIx4 vs the Q8400:http://www.anandtech.com/bench/Product/122?vs=89
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).