I'm throwing my support in for an ATI card since everyone seems to love nvidia (I love my Radeon 9800 Pro!). Most any ATI card these days supports dual monitors, and there's several in the ~$100 range that are very good cards.

A tip for those of you who use dual monitors AND play games (ignore this if you don't): set your primary monitor (1) on the right (yes right) of your secondary monitor (so the setup screen looks like "2 1" (look at Phil's picture above). The reason? When you play games, it tries to switch the resolutions and it'll crash games and force you to play in windowed mode because the second monitor doesn't want to play nice (stupid Windows feature). However, when the secondary monitor is on the left, it's not affected by the resolution changes, so everything works normally. Odd, no? (Don't forget to reboot after changing them. Logging out doesn't work, gotta reboot.)

And yes, I realize it can be a bit weird having their orders reversed. I have mine that way, and my primary monitor is physically on the left, while the secondary is physically on the right. So to drag something to the second monitor, I drag left, even though it's right. You get used to it. :) Now, I just need to get another LCD so my monitors can be set to the same resolution. :(