Before you shell out some of your hard-earner cash for a new video card, I strongly suggest that you look into whether or not you CAN disable your onboard video. Some machine from notoriously proprietary manufacturers are so restrictive that you can't upgrade the video at all. A chum of mine had a Packard Bell with onboard video and 2 different tech shops tried to disable it to install a TNT2 card last year. He ended up buying a new machine, since he;'d have had to have replaced the mobo, which isn't easy in a proprietary box, plus the video and processor...ticked him off royally, but at least he's happy with the new machine. ANYWAY....
Double check your mobo's manual, the manufacturer's website, here, wherever you can find info to make sure you can disable to video, and how to do it. That way you'll be all set to go when you finally decide on which card to buy.
For my 2 cents, I have to agree with AtreideS, the GeForce2 MX is a gr8 buy right now. I've seen them for about $125 Cdn ($80 U.S.). Since your system isn't top of the line (no offence intended) you really won't gain much by shelling out hundreds for video processing capacity that your machine can't use.
Good Luck.