We need some more information.
First, what type of 3.0 GHz processor is it? There's worlds of difference between a 800 FSB with HT and a 533 without HT.
Second, what Geforce 4 is it?
It would run games direct X8 and lower pretty good if it's a geforce 4 Ti 4200 or higher.
If it's an MX, it'll bottleneck everything and run like crap on newest games.
Games like Doom III would run at about 20 fps on 1024x768 with medium settings and about 60 fps at 640x480. My roomate has a geforce 4 so I've seen it personally.
Half Life 2 won't run so great.
The system would benefit most from a new video and some with a GB of RAM.