In reply to HarmM:
Well if you're only using 1 flash, the fact the 580 can be a master is redundant. You can control the 430 off-camera with the 7D's flash too (though this can struggle on bright days outdoors).
I started with a 430 EXII, then added a second with the Youngnou 622-C's to control them which is amazing. Plus it also means when you do want to go off-camera, you're controller doesn't have to be on camera.
If you want the extra power, I'd personally look at non-Canon flashes as they're considerably cheaper for the same GN.