Take the windows frame rate and chop it in half. Then thank Apple for having sucky OpenGL drivers.
(I'm sure it won't be that bad, HL2 and Portal are pretty old. But after SC2's terrible performance it looks like I may not be able to play any new AAA games in OS X.)
Based on what I've seen, it pretty much is that bad. Guess it'll be great for indie titles, and those with more powerful Macs than me, but for the moment my Windows partition will be staying.