Currently nvidia does not support gaming stretched over multiple monitors, with single or dual cards (sli or not).
Did the asus support talk about gaming or just regular windows use? As they are only right when it comes to windows use.
It looks like you'll have to go with the triplehead2go, and so the max you will be able to use is 3 monitors running at 1680x1050 each, but being only on a single 8800GT will mean that when you try to play at your 5040x1050 resolution, you will be pushing over 20% more pixels than a 30" 2560x1600 screen! Which is going to make everything but the oldest of games unplayable due to the lack of fps on the 8800GT. You will also have to be on vista as I'm pretty sure xp and dx9 are limited to screens with a maximum of 4000px wide.
If you are serious about going with three large displays like that you really will need a very powerful SLI/CrossFire setup (And on top of that a triplehead2go digital unit). Be aware that you will only be able to play games at a refresh rate of 57Hz at this huge resolution, as you are maxxing out the DVI cables bandwidth, so if you can't change/hack it to 57hz, you won't be able to play it at the full 5040x1050 resolution (if at all).
You should talk to mumm, as they have recently set up a triple screen system like you want to so might be able to give you some advice, you can see there screen thread here:
about15991.htmlDan