There are some useful charts here that you can use to compare the performance of different cards:
We can immediatly see that even a 7900GTX SLI configuration isn't going to match a single 8800GT. Plus you don't have DX10 support with the 7-series.
My advice is to buy a single 8800GT or 8800GTS G92. If you choose the 8800GT though, for god's sake don't get one with the Nvidia reference cooler. Lots of manufacturers fit an improved cooler, the Gainwards are the best, but apparently they aren't available in the US. Gigabyte, ASUS, Galaxy and Palit all have improved coolers fitted. eVGAs only differ from the reference design on the super expensive "AKIMBO" models.