A Radeon 9800 Pro can still handle most games, (i know, i have one) but Its seriously dated and will be incapable of playing next generations games, and is slow with even modern ones. I cant' play Half-Life 2 at high settings, or Medieval 2/Rome Total War etc etc though that may be down to my poor Athlon XP and slow, insufficient RAM.
I'd say it's time to upgrade, but with DX 10 now upon us you'll need to go PCI-Express.
Unfortunately for you that'll mean a new motherboard (the 9800 is AGP, There are loads of good ones, look in the Guides section or browse upgrade threads in the forums), processor, RAM etc.
You might be better off for gaming buying a new PC, or building one if your confident enough. The other forumites can give loads of recommendations if you want to build.
On the other hand, if you only play casualy or play older games it'll still be okay