Here's some good info on the relative performance of different video cards: http://www20.graphics.tomshardware.com/graphic/20041004/index.html
Note that AGP slots come in 1X, 2X, 4X, and 8X speeds. You need a really fast CPU and AGP slot to really get the most out of the fastest (and most expensive) video cards. For an older machine, I wouldn't look for anything above a midrange video card. I'd probably look for something like a Radeon 9600 Pro or NVidea 6600 card, though you should be able to get by with less. Of the two, the NVidea 6600 is one of the best values around right now. The only potential problem is it isn't specifically listed as being supported in the Myst Revelation documentation. The highest NVidea cards listed are the FX series, and here the Radeon is a much better choice. I didn't see any updates on this info on Ubisoft's website www.support.ubi.com
For what it's worth, I've been playing Myst Revelation on an Athlon 2100+ machine (1733 Mhz) with a Radeon 9500 Pro and haven't had any problems.
Huhm, I just noticed that a 32 meg Geforce2 card is supposed to be supported, even though 64 meg of video RAM is recommended. Do you have a DVD drive? This game comes on DVD and you have to have a DVD drive to play it.