steelx,
Right at the moment the 128 M card is sort of the "standard" level of memory......so that is the "target" I think you should head toward.
The ATI Radeon 9800 XT I have has 256M of ram...... but most applications currently don't really use all this. It was the top-of-the-line best rated card about 4 months ago,..... but the new generation cards are coming out now so it has slipped already ::).
Such is computer hardware..... gold today, junk tomorrow.

Of more importance than RAM is the "horsepower" of the video processing main chip on the card. These can vary all over the place even within brands.
Watch out for similar sounding card names when comparing prices. The Wizzard 9000 and the Wizzard 9000 pro may be VERY different performing cards... and the "pro" appelation doesn't always mean it is better. Caveat Emptor.
Look in the "hardware" forum for a couple of threads that have some graphs of benchmark performance tests of various video cards so that you can compare overall processing performance speed to price and decide on "bang for the buck".
If you ask about this video card subject.... you will likely get two basic "schools of opinion" about this whole thing....... "NVidea is great and ATI it terrible" and "ATI is great and NVidea is terrible". Up to you to decide if either "camp" is correct ;).
My guess is that ANY 128 Meg AGP card will GREATLY improve your situation over the onboard graphics you have.
OH.... make sure that your mother board supports an AGP video card before you buy one. The mother board has to have an agp slot available on it for you to install one. From your specs... it is likely that you DO have one.... but be sure.
best,
...................john