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It's like a mac user giving out PC advice, though. Anyway, I installed ATITrayTools, and there aren't any "extension limit" settings. [EDIT] Cleaning out 10.6 and installing 10.4 again fixed it. I'll bet it has something to do with the fact that 10.5 an 10.6 were focused on video playback, though I don't know how that affects OpenGL games.
Someone on the forum of Steam found a .dll file which solved the problem for me for Kingpin (as well as Quake II, and the laggy gameplay and cutscenes in American McGee's Alice), try this link : http://www.filefront.com/16696483/atioglxx.rar/ and put the file where your .exe files are located. It may work with other games based on the Quake 2 engine. this is not useful if you use older drivers like 10.4, as previously said.