gobi: if you want to play 1/2life and doom 3, then you gotta go PC, man. i love my mac dearly, but the games aren't there yet, although the mac gamers would ALWAYS welcome another into their market share. to give you an idea: bioware STILL hasnt' released NeverWinterNights. and no toolset. a YEAR later. that's pretty typical. when hte PC world get's expansions, we get the normal game.
That said, OS X is awesome, and if I were you, I'd nab a 10 Gig iPod and an iBook in an instant (AFTER TODAY! there will be new hardware announced!) So, after noon, when we see what's comin' down the pipe, maybe there'll be some better advice... maybe they'll drop the price on the powerbooks. in taht case, 12 incher all the way!
good luck, but like I said, the games just aren't there, so if you wanna game, be prepared to sit out on CounterStrike, , 1/2life. Doom 3 wil be out for Mac as well...
However, you can play UT, Q3,2,1, any Blizzard game in existance, Baldurs Gate 1 and 2, Masters of Orion up to 3, Stronghold, and quite a few others.