Lists 8800Ultra Tri SLI at max is 496w considering the ultras are overclocked the Tri 8800GTX will be less than that.
Nvidia suggests 1000w psu to cover is a number of ways...
1. Thats a PSU for the whole system so takes into account running heaps of other hardware
2. Cheap PSUs , if they say 750w will handle it everyone goes out and buys a *noname brand PSU that doesnt give a quality 750w at good amperage and clean then everyone start suing them for saying it would work.
And the reason your cant find figures on 8800GTX in Quad SLI is because they cannot do it. Quad SLI can only be done with Dual core GPUs such as 9800GX2 (hence done with 2x9800GX2's).
In the end youll probably get more value out of 2 new GTX280's or getting a single GTX295.