Another vote for 98se, if we're talking about an OS only for gaming. You'd be hard pressed to find a PC game that won't run on 98se, from the oldest DOS game to the latest shooter. The OS is pretty lean so you're gonna get good frame rates, etc.
Of course, how many PCs are only for games? Most gamers also surf the net, listen to music and watch videos, and even *gasp* work once in a while. For multiple tasks, XP is probably better -- what you lose in DOS games you gain in stability, memory management, and cool features that are good for gamers like 'driver rollback'.
Another factor to consider is the Microsoft 'sleaze factor'. DirectX 8 doesn't work with Windows95. If MS wanted, they could make DX9 incompatible with 98se. I don't think they'll do that....but I'd be very suprised if DX10 is compatible with ANY Win32/9x OS from Microsoft.
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sorry folk the joke has gone far long enought, time we get down to business hehehe, oh Im still joking!