It wouldn't normally work. SLI is locked for cards with the same Device #ID. You can have different Sub Vendor #IDs however.
The solution would be a cross firmware flash, since they are basically the same card, but this is risky. Backing up the original firmware is essential.
Or perhaps change the Device #ID with Nibitor and reflash. This is tricky since you need to have the correct checksum after you alter anything in your firmware. Soft flashing might work also, try using Riva Tuner first and hard flash it later if it works, but there are very slim chances of that happening. The nVidia driver might be harder to fool this way.
You should keep looking for another 8800GT, you should find one eventually.
I know that flashing the 8800GT (a reference card) to any reference 9800GT usually works without problems. Back up the original firmware! This is the easy way to do it.
