Spoken from someone who spent ~$1300 on video cards in SLI.
For high res's, benchmarks show memory is important so get the GTS 640 over the 320. Keep in mind it's easy to OC the GTS halfway to the performance of a GTX.
Ummm, I am not sure how to take that comment.
In any case it's my opinion. I didn't recommend SLI to the person who's post I was responding to. I did recommend the 8800GTX over other cards out now simply because I feel that the extra performance is needed in the newer games and certainly will for the next generation titles. There is no doubt in my mind about this. I had a single 8800GTX for a couple of weeks and I found that while it was more than capable of 2560x1600 in most of the games out at the time, there was opportunity for improvement which is why I added the second 8800GTX. I feel it was a good move for me as I've been very pleased with the performance of my system as a result of the components I've chosen.
For Dark Messiah and Oblivion, you almost NEED SLI'ed 8800GTX's to get good performance in my opinion.