I don't think NVidia has a card that will go in that machine... I think they're only selling AGP-based video cards for Macs.
I have the same machine as you -- A G4 Yikes! and I put in a Radeon 7000 PCI card -- BIG improvement. If you play games, you'll notice a phenomenal increase in quality and speed. Other than that, QuickTime movies are a bit smoother. On top of those two things, it allowed me to move my original Rage 128 card to a separate PCI slot and connect another monitor -- BEAUTIFUL! I don't know how I survived so long with only one monitor.
Don't expect window-resizing or window-moving to speed up dramatically, or at all. That's dependent on OS X, not the video card as much at this point, and even with the new Jaguar 10.2 coming out, we won't ever be able to take advantage of Quartz Extreme. We'd need an NVidia card or an AGP Radeon card... and I don't think NVidia will be releasing a PCI video card anytime soon if at all.
All in all, for the money, it was a worthwhile upgrade. Well-priced and it gave my system a little boost. I also highly recommend getting an ATA card (I got a Sonnet Tempo ATA/100 with 7200RPM 40GB and a 7200RPM 60GB) with a fast hard drive to put on there. BIG improvement over the internat ATA bus. And, of course, more RAM. I maxed mine out with 1GB (4x256MB) and OS X hasn't swapped memory out yet.
I know that seems like a lot, but for about $400, I breathed new life into this aging machine. Next on the list: processor upgrade, $300 for 500MHz. Not bad for a 20% increase in overall system speed.