Here's a video of one of my last ambient performances:
http://www.ev-b.com/vids/bluecube3.mov
(warning, 18MB file)
Some things I learned while gigging in that duo:
With two people you can help each other fill dead air while patches load, files reboot, etc.
Lots and lots of delay delay delay will cover over abrupt transitions, especially if a signal drops out unexpectedly, so have a delay online at all times ready to be triggered.
Loops are good for this too, if a patch or program fails (and it will eventually) you can start playing a loop to cover the dead air and no-one will be the wiser.
Minimize your cable routing. Try to get as much as possible of your signal and control paths to go through a fat firewire cable instead of a huge snarl of RCA's, instrument cables, USB's, MIDI cables, AC adapter wires, etc. We had two rigs connected by one analog audio snake and one firewire cable, and did all processing within each of those two "hub" rigs.
As far as gear you need, just get one good-quality two-channel D/A converter/sound card. Do a complete stereo mixdown "in the box", and send the two channel output direct to the PA. Use headphones to mix in the box and cue your loops/patches.
Video helps the stage entertainment a LOT and keeps you from looking too boring or dorky.
