Best Program For Live Performing?, Need access on the fly... Options

[LiseD]

I'm interested in using my PowerBook G4 667 as a basis for a virtual synth (VST or other format) system that will allow me to have the same sounds/setup available regardless of which MIDI keyboard I'm using. I play at three locations regularly (plus my home rig), and would rather tote my TiBook and a few cables than lug a rig (which doesn't fit in my car) to three locations that already have an adequate keyboard just so I could have my setups done in advance and have access to the sounds/splits/layers I like.

So far, I haven't abeen able to figure what software would work best for me. I would need to have access to a dozen or so sounds (set up in advance based on what we'd be playing), with the ability to change sounds, do splits and layers, and adjust levels on the fly with a minimum of fuss (maybe some kind of smart key setup). Somebody on another board recommended V-Stack, which I've tried, and seems pretty close, but I'm concerned it might not offer enough flexibility to switch or adjust quickly while I'm playing. The other recommendation was Kontakt, but I couldn't really test drive it because they didn't have an OS X demo.

Most of what I've seen seems directed toward recording, loop-based stuff or pre-fab tracks, but that's not at all what I do right now. I need to just play and improvise live. That other stuff is a bonus if the package already includes it, but not as its sole purpose. I'm running Panther with 1 GB of RAM.

Thanks in advance for helping a 'puter audio noob...