I think that the e8400 will be more than adequate for gaming and just everyday stuff. I personally have an e8400 and have to say that its great. I got it up to 4 ghz stable, but it was a bit more voltage than I wanted through it so i went back to 3.8.
e8400 paired with your gpu should be able to handle a majority of games at decent resolutiions. I don't know how much more the q8400 is, but if your not going to be doing anything too cpu intensive the e8400 should be fine, and you can save a few bucks.