The 430EX will be a good model to get - not as big as 580EX and still be able to bounce the light since it has tilt and swivel function, unlike 220EX which is a fixed head. The new 430EXII just came out a few months so you will find quite a few people selling the older 430EX. The 580EXII will be too big for your G7.
Yeah. +1 for the 430EX. I mounted my 580EXII onto my G9 before. It really look very weird and my wife was laughing at it saying the flash is even bigger than the camera.
For your G7, i think 430EX is more suitable unless you need the master function in the 580EX.