Because Nvidia's fastest solution : 3 x GTX280 is faster then ATI's fastest solution : 2 x 4870X2
This is because the single fastest chip is still the GTX280 and multi gpu scales less good the more GPU's you add.
Here is a link to a good review ...
No the main reason to go for Vista 64 is to be able to use 4GB of RAM and a couple of Video cards in SLI.
Or even 8GB of RAM, with DDR2 prices as low as they are.
Greetz
Aye 4GB of RAM or 8GB of RAM and a couple if video cards in SLI and there is NO CHOICE but to go for Vista 64.
But I guess they will tell you next that you don't need 4GB of RAM or SLI :p
Greetz