Win98 every time for me! I also run a dual boot system with 98 as my main OS. I use XP only for the latest versions of software that no longer have support for Win98.
Even using older GFX drivers for win98 I get a faster more playable GP4 than in XP on the same system. I have spent much time stripping down XP to save resources, and using the latest drivers but it will never match what I achieve in win98. The only way you may match it would be to install XP without any network/internet services (which is harder than you think), and even then this would force it to be a secondary system or partition for most users.
IMO there is nothing that XP can do for me that win98 can't. True! you need to know a little more to run win98, ie. drivers and configuration etc, but that's a good thing.. isn't it?
"Anything happens in Grand Prix racing and it usually does" (MW)