I recently built a PC for a friend with an ABIT NForce2 motherboard and a Radeon 9500PRO. He has set it up to dual boot between Win98 and WinXP. Everything works fine in WinXP, but in Win98 he's having a problem. It works fine with the motherboard's GEForce4MX graphics, but as soon as we insert the 9500 and load the drivers Win98 starts crashing before it finishes booting up.
Personally I wouldn't have bothered with 98 in the first place, but he seems to think he'll play some ancient games that don't like XP.