It really comes down to whether a driver works well with your games or not.
For the DirectX 9 Radeon cards, which up to X850 are all rather similar technology, perhaps Catalyst 6.2 (the newest). Though on the OpenGL side you will run into problems depending on the games you want to play. Bioware games are very troublesome on ATI cards and often need specific driver releases for best operation. KOTOR for example works rather well with Cat 4.2. Web searching can help there. On XP I like Catalyst 7.11 for the DirectX 9 cards because it seems to have really solid support for old games (7.12+ has a OpenGL overhaul that ruins this). Obviously that's not available for Win9x however.
For the DirectX 7/8 Radeons, I typically use a driver from 2001-2003. Catalyst 3.x or older. Radeon 8500 in particular is a buggy card and its behavior for things like anti-aliasing varies between driver releases.
On XP, Truform support ends at Cat 5.8. That may be the case under Win9x too but I haven't verified it.
The older Rage cards have far fewer driver revisions to try. You often need to edit the driver INF to get your particular card recognized because ATI played games with end user driver support back then. For the Rage 128 I have 6269, 6292, 7181, and 7192 drivers stored away for some reason. That's the only Rage card I have around.