Go directly to Logic Studio 8!
Has a ton of great synths, drum machine ( with step sequencer), samplers with tons of samples, apple loops, full track automation of just about everything, great filters, endless fx (delay, reverbs, distortion, compressors etc etc), great mastering. Comes on 5 DVD's - that should tell you all you need to know and ridiculously good value at £329. Hardly buys you a couple of good VI's these days...
Nothing comes close - lot to learn but my immediate feeling is that Garageband does not have enough depth or variety to create the kind of sophisticated multi-textural evolving professional soundscapes in modern electronic music.
Plus it JUST NEVER CRASHES and a new intel iMac will simultaneously runabout 100 plugin's and dozens of VI's without glitching or the need to track freeze...
(Reason is not comparable to Logic either - it's just not that well evolved)
As someone who has tried the lot - from £20K of analogue synths recording to analogue desks to just about every DAW & s/w package ever made I can confidently assure you that Logic 8 will not be bettered (except perhaps with Logic 9 in 2010)
Just take a look at who uses it :
http://www.apple.com/logicstudio/spotlight/
If it's good enough for Paul Van Dyk & BT it's certainly going to server you well!