You will be fine, yes faster CPU's are available and would give you higher frame rates but thats not what constitutes a Bottleneck.
You will be CPU restricted which people who dont fully understand the principles involved call the same thing.
Your CPU is only Very slightly slower tham a 6600, a Quad wouldnt make a differance per say, not unless it was a faster rated quad.
You could always overclock your CPU of course, but if you decide you want to get a better CPU then get a faster Core2 as games just are not coded to take advantage of a quad yet.
If you want to buy for the long term then there might be some amount of future proofing in getting a Quad but thats up to you.