So here's my (previous) point: start with two Canon 580EX II flashes (if they're in your budget) and see if one 580 on-camera can trigger one 580 off-camera in your typical scenario. Also, see if you're OK with manual power levels, or whether your style of shooting is better handled by ETTL (mine typically needs ETTL). That should guide you towards the solution, whether ST-E2 (if Canon triggering is OK for you and you want ETTL), 580EX II (i.e. a third, using one on-camera as trigger because you want more functionality than ST-E2), basic triggers (several to choose from), or ETTL triggers (at least two to choose from).