You don't need more than one graphics card. I have this running off a single GTX580 connected to a matrox Triple Head 2 Go digital edition. There is no fake stretching, you get literally double the viewable area as a person with a single screen.
Again, I can't really explain in words how much different it is to play with a 180 degree field of view. I was playing Alien vs. Predator last night and its just literally another game. To sound cliched, you're in the game, its incredible. Triple 26" monitors are nice but the bezels kill it for me. At triple 46" with no bezels you're looking at almost 100% peripheral vision coverage. That is was gives you an incredible sense of speed when in cockpit view.
I'll be posting detailed pics of the finished setup tonight with some how to's over tonight and tomorrow. Total project cost including the VR3, Triple monitor stand, projectors, consoles, misc was about 3,700 dollars. But I spent extra money I didn't have to because I'm a perfectionist. If you can do it, by all means do it, your gaming will be transformed. Not just driving games, but flying games, FPS shooters, adventure games, hell even Grand Theft Auto IV on this was amazing.
Word of warning though, once you do it a single display sucks. I'm not exaggerating. I went back to my 67" LED LCD last night to play some Halo Anniversary and even at that size I just felt like I was inhibited in view.