With the launch of Vista, it's gotten slightly more complicated.
If you're on XP you can stop reading here and go right ahead and follow the advice above.
However if you're on Vista or are planning to upgrade to Vista shortly, you need to be aware of the following:
1.) Vista has pulled support for Direct x8 cards. I think Vista basic can run with them, but Aero needs Directx9 and above. Make sure your card(s) are Direct x9 or above.
2.) If you have two monitors running off the same card you should be fine with this one, but if you decide to go for a third monitor, you need to ensure you purchase a secondary card that will run off the same drivers as your primary card.
3.) Ultramon is stable on XP, however when I tried it on Vista it was quite buggy. I'm not sure if it's been sorted now though.