I use a single 2.33Ghz C2D Macbook Pro for live sequencing, Soft synths and effects, with Logic Pro 7.2. It's a very reliable and stable platform for the stage. My audio interface is a firewire Edirol FA-101 which is bus powered and had 10 inputs and 10 outputs with very low latency, which meets my needs. I use an Evolution MK461 USB MIDI keyboard (nice and cheap with loads of knobs and sliders) + A Roland SH-201 for some USB MIDI, but mostly synth work.
I am just migrating my live sets to Ableton Live 6.0.3, as I think this software might aford me more felxability on stage, and may allow video sync. I'll be testing Ableton Live for stage work in April. In the studio I will be sticking with my MBP and Logic 7.2 :-)