Well from my experience a Q6600 will be the same performing as a E8400.
Although when it comes to single or Dual processing power. A e8400 would be the best performing bang for your buck.
But games like "Supreme commander" in which I believe uses Quad cores (Correct me if i'm wrong). Will perform better with the Q6600.
Reading a bit today and scavenging around the WWW. I found that people are saying a Q6600 is pretty much the equivilant of the E8400.. Until you get into software that will utilize quad cores.
There isn't really any downsides between these 2 processors. I'm pro E8400 but i'm biased.. Being that I have mine overclocked to 4.21Ghz and 21,000 3D Marks. It's a really forgiving processor for me.
Rone, Dr. and Clarke have the Q6600 and i'm sure they are based as well. But they have done amazing overclocks on those as well. So they can help answer your questions.
I'm slowly being left alone in the dual core world...
AMD uses a different archeticture than Intel does. And at the moment are losing the war in processors. They are good, cheaper, and I support them due to keeping the competition alive. But I wont use them for gaming because (No offense to anyone) But they are much slower, and have less cache on them as well.
I recently built a large HTPC server with a AMD Triple Core server. Hence saying I do support AMD. I have to help keep them alive.