If you're on Vista or 7 go to Computer, click System Properties in the bar along the top, click on "Windows Experience Index", then "View and print detailed performance and system information". Under the "Graphics" header you should see something like:
Display adapter type: ATI Mobility Radeon HD 3400 Series
That's my graphics card, so to update the drivers I'd go to the ATI website and look for a driver for the Radeon HD 3400. I can't remember how to get there on XP, or even if it tells you!