The only problem being that the 8800gt is still very scarce, and when it's in stock, it doesn't stay like that for long, while you can find the GTS much easier. It was intentionally done like that.
Here's some
Benchies to help you decide.. It doesn't look like you're sparing any expenses on that that rig, so the GTS may well be your best shot. But if you're looking to spare some change a 38xx series will suffice. The 3850 especially, as it's the same card as the 3870, just reduced Core clock. If you're overclocking you can remedy this easily, and add some extra omph as well.