I got up bright and early and got my Geforce3 Ti 200 at Best Buy on Thursday morning just a few minutes after 6am (and in some stores from what I hear, as there were over 100 people in front of me in line, I would have gone away empty-handed).
In some instances, it's up to 40% faster than my Geforce2 Pro 64MB was, though what I really bought it for was new features and visual quality. I've never seen 3DMark 2001 all the way through until now, which was interesting, but I really wanted stuff like environment bump mapping, and such. I bought Return to Castle Wolfenstein at the same time, and am hooked. Even with all the graphics features turned on, Single Player mode is very fluid at 1024x768 on my Samsung 19" monitor (by the way, I agree on the Samsung monitors, you can't go wrong with them for features/price).
I have a faster CPU than you, an Athlon XP 1600+, but I'd say with a P3-850 it still should make a noticeable difference over a Voodoo3 and it adds 32 bit color support in games, which is now a HUGE plus. Hope you managed to get one for your son, I'm sure he'd enjoy it.