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This thread I started my be helpful comparing cameras at high ISO. Some nice comparison shots with 5DII and 7D.
It sounds to me like we do very similar stuff, so I hope my advice will be helpful. I shoot the 5DII for this type of event (concerts, comedy shows) and I routinely use ISO 1600 and 3200. I have also used the 7D as a second body (also owned a 60D before I went to 5DII) and it 1-1.5 stops more noisy at ISO 3200 than the 5DII. I found that I needed to max out the 7D ISO around 1600 to get comparable files to the 5DII at ISO3200. In these types of venues the light is fairly low and the shutter speeds need to stay at around 1/125 or higher to get sharp shots. I use primes to shoot these events as I need the 1+ stop advantage over the 2.8 zooms. I have taken a 70-200 2.8 IS with me and used it some, but by far my most used lens is the 135 F2L (I can shoot 1 stop ISO lower with this lens and I get better seperation from the BG due to DOF). I also shoot a fast 50 and sometimes 24, 35 and 85. I just got a 200 1.8L for this same purpose (that is another whole discussion as Canon no longer repairs this lens), that should give you an idea about how serious I am about needed the stop.
Anyway, if you need low light, the 5DII is te way to go and the 135 F2L and possibly the 85 F1.8.
I don't have any issue with AF using the center AF point on my 5DII with focus and re-compose. I also got the EG-S super precision matte focusing screen which helps me see the focus as well. The viewfinder in the 5DII is also much better.
If you do go for a 70-200 because the venues you are working have more light than mine, consider the canon 70-200 2.8 non IS. It is very sharp and IS is limited in it's usefulness for this type of shooting. You can sometimes get a stop back if you are shooting at 200mm, but that is about the max usefulness unless they are still or you are going for motion blur shots with crisp BGs.
If you want perfect AF and low light, you will have to go for a 1D3s or 1d4. I don't have either, but I hope to get a 1 series body someday. I have been buying glass while I wait for the next gen 1 series to come out (and the current 1 series bodies to drop in used price).