professional 3d apps publish a list of "certified" cards for their app. these certified cards are usually nvidia quadro and ati firegl cards however they use the exact gpu as the consumer geforce and radeon product lines. the only difference is the these professional cards are provided with enhanced drivers and software support to ensure top level support for the pro 3d apps. some apps perform better with ati cards and some perform better with nvidia cards. the difference between the two is usually only noticeable to people who use a certain app *all day long*.

any current radeon or geforce will work with illustrator, maya, autocad, etc. but they may produce anamolies that you may or may not encounter. these anamolies are bugs that the graphics card vendor has no obligation to fix and most are bugs that are very infrequent or easily worked around.

so the "best" card can be found by checking the forums for your various apps and determining the latest/greatest ati/nvidia card, or sticking with a 9800gt (as you mentioned) and only upgrading if it has some deficiency that you cannot tolerate.