These are your options basically:
Newegg.com - Computer Hardware,Video Cards & Video Devices,Desktop Graphics / Video Cards,AGP 4X/8X
A good card will be choked by the 2400+, the lesser cards are not giving you alot of extra power, BUT are better than your mx4000, and won't be transferable to a new build, if you did one eventually.
Is it worth it? Thats a question only you can answer, are you planning a new build soon, or wanting to keep this around a bunch longer? The link shows the prices, you determine if you want to spend that much. A decent new build can be ha for about 500 or a bit less.
The ATI cards will most likely fit your needs for awhile, but need more power. Youy'll have to open the case and find out your ower supply Size, its in Watts, like 350W etc.
Thats all i got for you... Got more questions,. ask away.