I have both an A620 and an A640. I like the A620, but I think the A640 is far better. My wife is no photo expert by any means and she does fairly well with the A620.
The only comparison I can give you betweeen the G7 and the A640 is that the A640 fits better in my hand. It has a better grip than the G7. I do not own a G7 so I have not taken any pictures with one.
DesertEagle is correct in that the Canon housing will not support any auxiliary lenses.
I have the Canon case for the A620 and just got an Ikelite case for the A640. I have been underwater numerous times with the Canon case, but since I just got the Ikelite case, I have not gotten it wet yet. The Ikelite case is a more substantial case for sure, but I took the Canon case down over 100 feet with no problems.
Sorry to ramble. If I can help with any other questions, please PM me.