If you're going from a G3 iMac to a Macbook, then you'd better strap yourself in. My wife has one and it can do pretty much anything you want except do games and 3D apps.
As far as Logic Pro is concerned. Man, if you think cubase is complicated then you'd better just stick to garageband lol. Garageband is everything Logic is not. Intuitive, straight forward and VERY LIMITED. Logic is wonderful, flexible, great plugins, great features, great editing, HUGE LEARNING CURVE. However, you should stay tuned because the rumour is that the next version of Logic will be released around the same time as OSX 10.5 Leopard. Word on the street is that Logic Pro 8 (or whatever it is Apple will call it) has been rebuilt from the ground up including a completely redone Apple GUI and workflow.
You mentioned that you wanted to use Logic for live performance. What exactly are your needs? What type of music? I think that if you're recording, composing and are willing to put in the time and effort to really get to know the software then Logic is the best choice on the mac. OTOH, if it is strictly for live performance, then there may be better choices instead of Logic. I use a great program called RAX. It is a virtual Audio Unit rack that I can load up with virtual instruments and FX. It can playback midi files and organize them into setlists. It even has a visualizer built in so you can synchronise and integrate visuals via a projector.
Rax has been discontinued at the moment but the developers have stated that they are in the final stages of selling off the software to another company that will continue development. For details about RAX you can check it out here:
http://plasq.com/raxTo keep an ear out for who will be continuing development of RAX:
http://plasq.com/component/option,com_joom.../limitstart,96/