Both gpus share the same Memory 256-bit bus resulting in 128-bit memory bus and 128MB of ram for each. Basically you have 2 6600GTs without the latency problems associated with SLI. Still for the same price 6800GT gives it a whooping in real world gaming (not synthetic benchmarking).
With introduction of X800XL, even 6800GT will be under fire. This card also only works with the Gigabyte SLI board and the 2 must be purchased together. Basically, this card isn't a good option for anything as it costs a lot, performs rather poorly for the $$$, and requires an SLI board, further increasing the price of the bundle.
Review