Back to the question, I forget the details but don't you need to make sure your board supports AGP 2.0 and there was something about the voltage of the slot as well.
Anybody remember the details???
And who cares which is better, ATI/Nvidia. I've run both and they've worked great. Can't remember the last card I bought that sucked from either one. I went from an ATI 9600 to my current 6600GT (AGP BTW). Both cards worked/work great for me. The 6600GT just happened to be the best mid-range card AT THE TIME. It changes back and forth. I remember when some of the old FX5000 series of cards were getting owned by ATI. It changes constantly. Go with whoever has the best product for your budget/needs. In linux's case he's right, NV cards do generally work better with Linux. Their drivers are more optimized for Open GL, something Linux natively supports as opposed to DX due to licensing issues.
But Pauldh also has a good point. Generally in Windows, yes ATI favors DX while again NV favors Open GL. However, the differences usually aren't that big enough to make the games completely miserable to play from either standpoint.
"Of course that's just my opinion...I could be wrong."