I don't think a Geforce 6600 GT is much more expensive, and that card is quite good, and will run games at rather high settings, compromising on resoloutions though! Plus it has won many awards
It's probably the card I'd go for now if i had money to buy one, and would definetly be better than the Radeon 9800 Pro 128mb I have on here at the moment and strangly I'm not usually a Geforce person... yet it is much better than ATI's offerings at the moment in mid range
n hehe, i uised to have a Geforce MX 400 (yes the original) before I mae the big step up! Lasted me ages!
As for your computer, it is all ready pretty good. You wouldn't notice a big difference adding another 512mb of ram I found, but the graphics card will give games a big boost trust me. Maybe update to a Athlon 64 sometime though, that'll give you more of a future proofing
Or if you wanted to get more performance later, try and get a SLI motherboard, which basically means you could add 2x geforce 6600 gt or 6800's to get much more performance on PCI Express. Though a normal AGP card would do fine for now