Well, it depends on your PC. If you have low specs, then I don't think you will see any improvement at all. However, even if you have a high spec PC, you will find that the GeForce4 MX will perform only slightly better. If you really are looking for smoother and faster framerate, if you are interested in gaming of course, then you should go for at least a GeForce 4 Ti4200. You will then experience a huge improvement in frame rates, but you will only experience this with a high spec PC.
I managed to pick up a GeForce4 Ti4200 for £60 and I increased my 3DMark2001SE score from about 2000 to 7000, which could have been much higher if I didn't have a Duron 1.3Ghz.