our basic visuals consist of a laptop (PIII 800, 512MB) and a projector..
s-video out of the laptop to the projector, so can have two seperate desktops screens on the laptop (dual monitor basicly)
projector screen has full screen winamp visuals, laptop screen has winamp windows + associated plug-ins setups etc...
you can trigger visual changes by "hot keys" on the keyboard, we just use the in built slide changes (fade way, fizzle, slice etc.)
its not exactly mixing visuals, but u can mess about with the settings of which ever plugin being used , or like me, just program in loads of presets and let the thing change automaticly after 20 seconds or so - that way, no one has to VJ..
you can get the visuals in beat time as well.
winamp can be set to play the mic input through itself.
The visuals take their instructions from the music's frequencies (i think...), the frequencies that go through it are the ones coming from the mic, which is the music off the sound sytem - that bit I espcially like.
is simple, yet quite effective.