paopao -- I don't know too much about the Radeon LE, so I can't give you a specific opinion about whether you should upgrade. However, I did upgrade from a GeForce DDR to a Gainward Ti4400 (not 4200, but should be close). Also, not sure how you feel about 3dmark2001, but this will give you a sense for my benchmarks (all at standard 1024x768). My system is an old PIII 800 (o/c from 600), 256mb PC100 RAM, etc. (soon to be upgraded). I went from 2500 3dmarks to 6000+ with the new card. I can turn on 2x AA with a minor hit, and 4x AA drops me only to 4000 3dmarks. Overclocking video card does nothing for now, because I am so processor limited.
Should you buy it? I upgraded now because the fan on my GF DDR died and I was getting eratic performance, otherwise, the card is too much for my system (until I upgrade). You'll see some improvement and it will only get better if/when you upgrade the rest of your PC.
Good luck.