I want to buy a PC and run three monitors from it. I need the desktop to span across all three.

Purpose / intended use

This is for pretty demanding gaming applications so I need optimum speed, resolution, and quality in general. I'll need to work with up-to-date graphics hardware. Not necessarily absolute top-of-line, but let's say good, 1 GB graphics cards less than two years old.

I need to be able to expand the window containing the game across all three monitors.

Previous questions

I noticed this question here and I think I learned the answer — install two video cards and take advantage of the fact that most video cards have at least two outputs. So you'd run monitor 1 from the DVI output of card A, monitor 2 from the VGA output of card A, and monitor 3 from the DVI output of card B, perhaps.

However in that question there was a fair amount of discussion of other options, situations, and topics so I just want to confirm that two cards is the right way to do it.

Mixing brands?

Also, do they both have to be the same brand, like both be ATI or nVidia? I know ATI has that Catalyst config utility, so if you tried to mix brands would that clash with the equivalent nVidia config utility? This matters to me because I already have a good ATI card I like so if I bought a machine with an nVidia card I don't want to everything to get all fluxed up.

What about three cards?

Graphics cards aren't terribly expensive these days. Would it make sense to install three cards to get the higher-quality DVI output on all three monitors? Has anyone tried this? Could it work?