I say go with the two 8800GTS 320 cards.
Of course as Mazair points out the 320MB would be a problem in some more modern games especially at higher resolutions, but not all games will suffer from that. Games that are straight console ports, for example, tend not to use as much vRam since they were designed to accommodate the limited memory on the 360 and PS3. Not all games need so much vRam that two 320 cards would run slower than one 512MB card. Just depends on what you play.
True but on resolutions like 1080p,320MB RAM isn't enough