I would go with the one GTX and get a SLI board so later you could get two of those. Plsu I am almost sure that a GTX runs almost as fast as two 7900gt's but I could be wrong. Check benchmarks on this website
I appologize. I was under the impression that the 7900 GTX was like all the rest of the 512 cards out there, which is actually 2 x 256. As in basicly two cards stiched together, with two fans. I had not taken a close look at a picture of this card.