Just make sure when you buy a new video card that you buy one that is suposed to run in a P3.
You might think that is silly, but the AGP specs have changed since the TNT2. The cards you buy now require more power and will not work in an old motherboard that is not capable of providing the juice it needs. (voltage went up from 3.3 to 5v I believe).
That AGP upgrade didn't even seem to be enough for today's cream of the crop cards (ATI 9700 & nVidia Gforce FX), 'cause these cards also feature a molex connector on them because the AGP slot isn't able to provide enough power to it - again.
That should be solved with the new forthcoming PCIexpress technology.

Some motherboards can be damaged when the wrong type of AGP card is inserted, and they carry a warning sticker.