We spotted a nice deal on Alternate and it looks like you will be able to get yourself a Quad SLI system for only €358.
To be honest, this is still a DirectX 9 Quad SLI and if you buy two cards you will end up with 2 GB of memory, not that you need it. You are safe with DirectX 9 cards as there are still no DirectX 10 games around and it will take a while.
So get these cards while they last and I am sure that two 7950GX2 are faster than a single 8800 GTS 640 MB and they cost about the same. It would be fun to compare them.
You can find them here.