A couple of us on this forum have picked up GeForce4 MX cards with 64mb of RAM for $10-15 on eBay in the last month or two.
I just replaced the stock GPU in a 400mhz Gigabit Ethernet with one. Works fine.
They made versions with ADC and VGA, and ADC and DVI.
I was one of them, too. It's a cheap, silent, easy upgrade, that makes a big difference IF you're running a large-ish monitor. You've already got Quartz Extreme support, and you're not going to get Core Image unless you buy a Radeon 9800 (only compatible card that can do it), which is not worth the money for that machine. Unless you're planning on gaming, or you notice slowdown in normal tasks (Exposé etc), I wouldn't bother with the upgrade, but it's not a bad/expensive way to go to get a Geforce4MX, especially the DVI+ADC model (best monitor compatibility). Obviously, the 4MX isn't a gaming card, but for day-to-day use, it's easily the best power/silence/price balance available. If you want OS9 gaming, the best option is the Geforce4Ti; if you want OSX gaming, the Radeon 9800, followed by the 9000, 4Ti, etc etc...but I can't imagine setting up your machine for gaming. By the time you upgraded it enough to make it a good (OSX) game machine, you'd be better off just buying a whole separate faster machine to begin with.
Note When I bought mine, my old card was a Rage 128 Pro, which wasn't even Quartz Extreme capable--so the upgrade was a no-brainer. When I got my Quicksilver (with Geforce2MX) a few weeks later (free), I swapped in the 4MX, but wouldn't have paid to upgrade from 2MX to 4MX, even though I can notice a small speed difference between the two in Exposé.