If it's 500mhz, then your computer sounds new enough to have an AGP slot. If so, then you won't have much issues, pretty much any AGP card will run your old DOS games fine. If you're getting a PCI card, I'd say make sure it has at least 4mb of video ram. My personal preference for the video chipset is ATI and Matrox though on this forum, I hear a lot of positive remarks about S3 products as well.
My Gateway 2000 system has an 8mb ATI Rage video card which seems to run with anything that supports Mach64 and runs all my old games like a charm.
Since you're running a GUI OS, I'd shy away from anything with less than 4mb. Anything with less memory will force you to pick between low resolution or low colors when in Windows.