I'm kind of anti-XP just because the whole activation thing seems like a real pain. I change components around often enough I just don't want to deal with it. XP doesn't offer much that 2k can't do, although I've gotten the impression XP is somewhat faster with games. This might just be because XP is a consumer OS so more effort was put into optimizing video drivers for it.
Back when I switched from 98 to 2k, I found that the 3dmark scores were something like 10% slower, that was on a Geforce2 MX. If you play games much, want the best possible speed, and don't mind putting up with the flimsiness of win9x, then I think the best arrangement would be a dual boot. Use 98 for games, 2000 for everything else. If you do it carefully, you could get the drive letter of your Games drive mapped the same under both OS. Use FAT for the games, NTFS for stuff you don't need under 98. Install each game under both OS, so as to get the registry and such set up properly for both Windows installs.
Of course, dual booting is a hassle too. Assuming XP is as fast as 98, then XP would make that unnecessary even for the speed obsessed.