I don't know the answer in terms of games, because I don't know how most games access the RAM, but in terms of bandwidth ddr3-1600 >> ddr2-800. The ddr3 would never perform worse, unless I'm confused.
http://en.wikipedia.org/wiki/CAS_Latency
Their latency (the time to get the first bit) would be identical, but the higher bandwidths of the ddr3 would send data at a much greater rate. A well designed program should therefore take advantage of this by reading clumps of data from RAM rather than scattered bits of data all over.
The questions that I don't know the answer to is "how much does increasing RAM speed affect game performance?" and "how well are games programmed to take advantage of fast RAM". My guess (unsubstantiated, mostly), is that in terms of FPS, there will be little difference using either of these on a Intel c2d, as the large L2 caches seem to make them relatively resistant to increasing RAM speeds.