Although I haven't been able to find benchmarks comparing the two setups, would you think there is a noticeable performance diff between the two? Disregarding dx10 capabilities the 8800 has, which upgrade would I be better off with?
Some things to keep in consideration:
-Power consumption (only a 500W PS)
-Value for my money
I would say that the 8800gts(640mb) would give you better performance, although I haven't been able to find a benchmark comparing that specific setup.
Particularly the superclocked version. Others may have better information about specific benches and such. I know in general that SLI'ed cards will not perform well in every game, as only some games will take advantage of both cards.