You can get the official system requirements here :
The Witcher Official Website . Make sure that when you get the game you patch it, because there are some speed & loading issues with the engine itself.
To give you an idea, I'm running it with most settings in medium to high mode, and some of the ground cover and irrelevant stuff turned off, and it still looks fantastic. My system is 4 years old (though the video card is only 2) P4:2.8 GHz with Hyperthread, 1 GB RAM, and a GeForce 6800 Ultra 256 MB, running on XP. A new gaming rig like the one you mentioned shouldn't have any major problems, except the obvious ones.
The obvious problems are a) problems with the game engine itself, b) not everything deals well with 64bit, c) Vista doesn't like games, and d) GT spec cards are the lowest in the Nvida line. If you have the money for SLI, why are you scrimping on the card your putting in?