Awww.. please don't stick someone with a RagePro card, if you can help it. Chances are, if you need a faster card for gaming, whatever 64MB card that is in that system now, isn't going to be fast enough either. It's probably a GF4 MX or something, or a GF FX5200 or 9000/9200SE, with only 64-bit memory. (Just a guess.)
I had to suffer^H^H^H^H^H^Huse a RagePro in Win98se and W2K for a while, and .... let's just say the drivers left a lot to be desired, ok? At least with whatever 64MB card is in that system now, it probably has up-to-date drivers available.
Just the opinions of a former ATI RagePro sufferer.
(On the other hand, on my PII 450 rig, it did play DVDs pretty well through the TV-out, and supported 100Mhz AGP bus speeds. But it was slow as dog * in 3D.)
Edit: Bump up the system RAM on that machine to 192MB if you are planning on installing XP, definately.. any less, and I wouldn't recommend XP at all, stick with Win98se. RagePro is only supported by out-of-box drivers in XP, and a Voodoo3 even less so. If you could "downgrade" the OS to Win98se, I would say throw the Voodoo3 in there, it should make a nice UT gaming machine.

But if sticking with XP.. hmm, honestly, the person would be better off keeping their GF4 MX. Perhaps you could convince them to pay you a few buck for doing the upgrade (and charge a few bucks for the RAM), and you could put that towards saving for a Radeon 9600 or something?
In any case, if you ever do use that RagePro in Win98se or WinME, PM me for some of their later Beta drivers that I snagged before they took them off the web. I've had better luck with them than their "release-quality" ones.