You will probably want to check with compaq (website first, call only as a last resort) to make sure you have normal pci slots on your motherboard. Also, If you dink around inside your PC they will get all pissy cause they think they're the only morons qualified to do it.
Assuming you haven't been screwed by compaq's love for proprietary hardware, most of the soundcards talked about on this board will be good.Look around at the other threads. You can pull alot of weight with that p3, even at 500mhz.