I´m afraid, if you want a wide range of choices, you will have to get Windows. The Mac just isn´t a gaming platform at all.
If you install Windows and your friends are playing Team Fortress 2, get the "Orange Box" on Steam! Can´t beat the price for the amazing content you get. You have to like first-person shooters though.
For eyecandy, check out Crysis.
Mac Games:
I still haven´t decided if I really want to buy a copy of Windows just to play a few games.
I intend to try out "Eve Online". Apparently there is a (Intel only) Mac-version of that game.
World of Warcraft is good, but it´s a dangerous time-sink if you get addicted! I´m not joking. It can get out of hand with some people.
I´ve been looking around a little. If you just want to see some pretty graphics, you could also download this Prey-Demo:
http://www.apple.com/downloads/macosx/games/demos_updates/prey.html
I´ve also seen that there is a mac-version of "X3", which has amazing graphics. But unfortunately there doesn´t seem to be a demo available and the game is not very accessible.
That´s all I found for the moment. I would like to hear more from others but I´m afraid, I will have to install Windows after all.