Realtime Tweaking, Which software is good for live playback? Options

Hey Everyone,

I want to start producing, mainly for the purposes of live playback. Mainly in a band setting, but I would like to eventually move towards thowing down some beats solo. I would like to be able to use software instead of hardware, because I tend to find it easier to work with visually, plus there seems to be a bit more that can be done digitally.

My question is, which programs will suit this purpose? I definitely need the program to be versatile, in that I would want to be able to change stuff on the fly. Maximum tweakability is an asset.

I've been told that Reason is a good program for both loop based, and sequenced programming. Is this true? Would anyone trust this program enough to use it for a live show? I also like that it can do what thousands of $$ of hardware can do, and it'll all be packaged on my laptop.

Oh, one more question. For the software that I would be looking at, would a G3@600 be sufficient for live, or would I want to go with a G4@550?

Thanks for the help, right now I'm completely clueless when it comes to this stuff, and I don't really trust the guys who are trying to sell me this stuff...

Ryan