6600GT doesn't come in 256mb... and as fast as SLI is I seriously doubt you can just double the specs of each card to find how effective it would be. Also, a PCI express connection certainly does not mean that the card will be twice as fast, it is referring to the amount of bandwidth, I believe. The reason I won't get an X800XT or an 6800 ultra is that the 6800GT is a much better deal than either, and I am pretty sure that SLI 6600GT's beat a 6800GT, but I am not sure. Does anyone know?
theoretically the Nforce 4 would perform better. The PCI Express has more theoretical bandwidth than the AGP. However, nothing today can throttle it enough through the north and south bridge to make a significant difference.
The 6800GT will outperform two SLI'd 6600GTs for most purposes. there are some situations where the SLI'd cards would outperform even a 6800 Ultra. I'd imagine that the SLI capable motherboards, when they appear, will be restrictively expensive anyway, so I'd go for the 6800GT AGP for now ;)