Either get the new MacBook Pro's, or higher-end iMac's, and you can run games at max settings in native resolution. All the games released by Steam for Mac thus far (I own all of the top-sellers) run at 1920x1080 max settings (and they were set that way by default.) sometimes you need to check the texture setting though, it might set it to medium by default, but you can bump it up to high/highest.
If you get an iMac, just make sure to examine the screen closely, and watch for any defects over the course of 3-4 months. I believe Apple has since fixed the screen defects issues, but I'm one of those who were unfortunate to have picked one up with major screen defects. I haven't taken it to an Apple store yet because I would have to backup a LOT of data first. I believe the only fix for my problem is a complete replacement.
My brother and I have been gamers for as long as we can remember, and both of us do our gaming these days on Mac's. We both had picked up new high-end gaming laptops (Asus core i5 and Sony Vaio core i7), but found that they run very hot and are annoyingly loud, to the point of being distracting. Our Mac's run cool, and silent. I am actually quite surprised how well we can game on the new MBP's and iMac's.
Now all Apple has to do is get a handle on quality control, and bump up the video ram.