If you have WinXP SP2, you can install Boot Camp, dual boot to WinXP natively, and run any game that a PC notebook with the same processor, video, and RAM could play.
I've never played any native UB games in OS X, but reportedly Quake is fantastic.
If you're into a brainy kind of game, you could look up TubeTwist. Not sure if it would run in Rosetta, you'd have to try.
I also know there is a newer version in the works, TubeTwist: Quantum-Flux Edition but for now only the Windows version is ready. I would imagine that Garage Games will once again do the port to Mac OS X.
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