You cant crossfire ATi cards on an nForce motherboard. You'd need an X38 or X48 intel chipset for that (for 16x16) or you could buy a cheaper P45 (for 8x8) There will be no difference despite the slower slot speeds with the P45.
You'd have no problem running a 4870x2 on an nForce board, just you wont be able to double ATi cards up.
Aslong as the GTX is the same core type (eg. G80) then it will work. It doesnt matter what vendor they're from. The 8800GTX only comes in the G80 flavour anyway so whatever card you pick up, it will work no problem.
The power supply should be fine.
---------- Post added at 19:25 ---------- Previous post was at 19:23 ----------
Quote:
Originally Posted by chill
Looking on eBay, I could sell this card for around £100 or something.
Would spending another £100 on another 8800GTX be better than spending £200 on 2x ATI's?
Would I get the same/better/worse performance with that budget on ATI or nVidia?
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Personally, i'd sell the GTX you have and buy a 4870x2... you'll get much more longetivity out of it. Even in games that don't support crossfire, you'll still see a better improvement using one core over SLI 8800GTX's