I think it makes more sense the other way round.
Crop for extra reach, full frame for the added quality.
I have a 40D and desperate to upgrade it, I also have the 17-55mm but my 70-200mm 2.8 gets more reach so it makes sense to sell the 17-55mm and get the 24-105 kit lens on the 5D with the extra 3 stops over my 40D you can use the 24-105mm in the same way as a 24-70mm on the 5D (obviously you won't get the same DOF) but its a much better length. 70-200 works out at 112-320mm on a crop camera so your only loosing 7mm instead of 42mm on the 24-70.
Means you cover 24-320mm then chuck a 1.4 or 2x extender and your covering up to 640mm
I don't know about you but I much prefer to use two camera bodies a crop with the 70-200mm with an extender if needed and a grip and a full frame camera with a standard lens. Perfect wedding kit, also events! I shot last years British Touring Car Championship with this set up and it worked fantastic. A 5D taking pics in the pits and of cars being sorted pre race with drivers and spectators etc then crop with the 70-200 for the action.
With this respect you may not need to go the full hog and get a 5D MKIII and get a MKII instead. It would save around £1500. This also means you are ready to upgrade the camera when the 5D MKIII comes down in price and the 5D MKII price will stay strong in the second hand market place like the original 5D.
I know the 9 point system is not perfect but using a standard lens doesn't need a huge amount of AF points usually I just use the centre. I wouldn't be chasing subjects with it, more still objects whereas on a crop with a tele I would be shooting moving objects like cars etc and the AF on the 7D is more than adequate with its group AF selection and the IQ is also the best on a crop camera atm.
Just my two pence.