Getting better performance out of Half-Life based games on MacBook

If you have recently updated to OSX 10.6.4, then you should boot up your mac with your OSX DVD and reinstall. You shouldn't lose any data or apps (though a backup would be advised).

Make sure you don't install the 10.6.4 update again (duh, but I did the first time round :-p)

You can find OSX updaters here which won't go all the way without asking you.

I believe there were issues running the games following the latest OSX software update - Steam recommended that you not update.

I've had similar issues though generally a restart of steam is the best solution for me.

While there are likely other issues causing the problem, have you taken a look at Tweak Guides? The Half-Life 2 optimization guide might serve as a stopgap until you can upgrade.