In order to keep the peace...
First of all, most games that need killer cpu depend even more on the GPU, so the difference will be marginal.
Second, if the game does need a good cpu (like for example Starcraft2), then it will depend more on the game coding than on the model itself (same as when nvidia vs ati change the winning positions on different games depending on the drivers).
I suggest you think more on how long you want that pc to run for you with the new cpu, to make sure you want to buy now and not later.
Its hard to really guess how long a cpu and gu will be powerfull enought to run games great, but since most games are done first on console hardware now...
You probably can still be working on a E6600 Dual core or a Q6600 untill the new generation Consoles come out (my Desktop actually uses Q6600 and only slows down starcraft 2, because it runs better on my laptop using a i5-2430M).
If you dont want to wait however, id probably go with the I5, as Intel is just more popular, so probably compatibility with its architecture is probably better.