Dude... I think I ran into this before with the game... don't pay attention to the installer itself– it's installing some HUGE files and WILL TAKE AWHILE. Go watch your favorite discovery channel show during install and let it do it's thing... believe me, it's working.
No, really... I'm not insane– I own a Mac, damnit!
Have fun with the game... and I hope you're not spooked easily!
(If you like undying I think you'll LOVE Doom 3... dunno if it'd work too well on your setup, tho... works on my MDD 1GHz G4/1GB RAM, Radeon 9800 Pro, OS 10.4.3... blah blah blah...)