You probably should try to add another 256 MB stick of SDRAM to your system (assuming it has a single 256 MB stick now.) 512 MB of RAM is really needed with WinXP and the game will make use of extra RAM as well.

However, the vid card you listed is really ancient. If you are ready to buy a new card, I would not look at less than an ATI 9600 level card (and not the 9600 SE which is a 64 bit version of the card.) Avoid cards with SE or MX in the name. I'm not sure what Nvidia cards are good for RTW, but I would pass on all the MX cards, the 5200's and the 5600's (the 5600's are having serious driver issues with RTW for a number of folks.)