If you're just playing Quake 3, Halflife, etc...there shouldnt be a problem. I ran both those games on my P2-250 w/64mb RAM and had no problems at all...
Anyway, doubling the RAM is a good start. That'll take care of most speed issues. Beyond that, defraging the harddrive is good. I wouldn't worry about upgrading processors or graphics cards just yet...most games run fine on P3 systems.
I have the same graphics card you do, and it handles pretty much everything. Nvidia updates thier drivers quite a bit, so have a look at thier webpage from time to time....