Hey Octo, the reason you are getting choppy graphics is due to your gfx card. A gf 4mx is pretty much a glorified GF 2. You could get a ti4200 or even better an ati 9600 for about 100 US. I think you need at least a GF3 to play smoothly. And chances are, you will have probs with HL2 also. You might look into upgrading.
I would have to recommend against getting a top of the line card, unless you are one of those people who have to have the best of everything. If you want some info on good gfx cards you can go to www.guru3d.com they are good people there, and very knowledgeable. Hope this helps.
EDIT: I just remembered, HL2 is being shipped with ati's 9800 cards i think. Perhaps you could save up now, and kill 2 birds with one stone.