go out and buy all the CD's. you won't be disappointed. just rip them into itunes. also, if you're more interested, pick up the Led Zeppelin DVD, The Song Remains the Same, and How the West Was Won. that will give you some of their greatest live performances.
i wouldn't buy Mothership off of iTMS (it's on there right?) as it's kind of a dumb compilation.
BTW, you should get the "Complete studio Recordings" version. that's the newest remastering.
-the biggest Led Zeppelin fan ever
EDIT: john, you lucky SOB, how on earth did you get tickets??? i hope they tour when Robert's done with his stuff.