Good question, had me thinking for a bit as I like to listen to many differnt lineups...if it's my show here's my ideal setup:

Lead/Rhythm guitarist (Me)

Singer (preferably NOT me) - who also plays guitar but isn't always active on it...ie: he puts in harmony, rhythm behind a solo...etc and shuts up on the songs where you only need one guitar (I play a lot of blues and can often get by on my own.)

Bassist

Keyboard player: (gets a 'fuller' sound, like Rush, Genesis, Theater, Grateful dead; Yes, GnR)

Drummer