I didn't find the AF on the 20D all that bad. In fact from an accuracy PoV I'd say it was pretty much perfect, I very rarely had an OOF shot. This was typical, and was with a 400 F/5.6 + 1.4x TC:
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The 7D obviously has features that make some things a little easier though. I like the zone and expansion options. I'm also not sure about the macro servo focusing, I've got a Sigma 150 macro which the camera seems to think is a Canon 180 macro, so I guess it will work but I haven't tried.
The resolution, AF options, level of customisation, bigger viewfinder, liveview, high ISO performance and wireless flash were all solid arguments for the 7D in my mind. It's MUCH more camera than the 20D, no doubt about it, but I don't personally find the AF accuracy/reliability any different (and perhaps a touch worse when a F/8 lens combo is used with the tape trick). If I can justify keeping it the 20D wont be going anyway, I don't feel disappointing using it straight after having used the 7D.