I never, ever use ETTL....always manual. The 20d and the 580EX II will be fine, just bounce the flash. The only thing I change during a wedding is the ISO and shutter speed, I usually keep the aperture at 2.8 unless it's outside or shooting formals where more DOF is needed.
I find that I have to disagree with you on most of the points that you make here.
EttL is fine if you are familiar with how it works I personaly have found it works very well you just need to know when to adjust fec
Staying at 2.8 limits how you shoot your wedding I dont really understand your thinking here.
The only sensible part of this post is the advice to change ISO and Shutter speed.
Why buy the latest flash unit and then disable a large chunk of it ? 
Wilky
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