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.
@ baustin612: Yes, it will hold back the card a bit but I would not worry too much about it.
At that resoloution go for the 1Gb card but also consider the GTX260 (much the same performance as the HD4870 but from Nvidia) or the GTX275 which is somewhat faster than either.