I have a g92 8800 GTS and have found the 3d modes to be pretty good, but the 2d performance is pretty sucky for some reason - although there are work-arounds like using a compositing WM.
It's very easy to configure multiple monitors with the nVidia drivers and I've heard that it's more tricky with ATI, but can't confirm.
AMD/ATI have claimed that they're going to open source their drivers which should be a big help, but AFAIK they haven't done anything about it yet.