Do not buy a multi-GPU card, support is simply not good enough for them and the 8800GTS is more than enough for everything out there. You're better off saving your money. More and more developers are optimizing their game engines for consoles which only have a single GPU, so if you ask me, multi-GPU support is only going to get worse.
If you need a 1gb 8800gts, you can get one I think.
Yes you can, both Gainward and Palit make a 1GB GTS so if you play at a high resolution, the 1GB of VRAM will increase your performance.