I haven't built such a system. However I have looked into such things. Two Nvidia 7900 512mb cards (in SLI mode) can give a better performance than a single card, it depends on the game. The performance using two cards can be anything from the same to 1.5 times better, maybe more. I'd recommend the 512mb cards, although not many games currently request 512mb of graphics RAM, in the long run it might be a better idea. As for saving money, if you can wait a litte while for the graphics cards there is a price drop due soon, and one 7900 512mb card is good, without the second card. So perhaps wait a little while, and purchase one 512mb then purchase the second a little while after.
Good luck building the system.