Up until the 5D MK III, the 5D MK II with center point was the best low light AF body Canon made. Its wonderful. This is a well known fact. its also well known that the 5D MK II does not do nearly as well with other than center points, don't even bother with them.
But my main complain: Does the focus of the 5d2 really suck balls in low light (sorry for the words)?
I know how good High-ISO is and i love the body and the button layout but i never handled one in low light.
I loved the 40d, I had five of them, and have had two 5D MK II's, which are a much better camera. I also noticed that the 2nd 5D MK II had a much lower sensor noise than my first one, I think Canon's sensor manufacturing quality has improved over the 4 years its been out.
The 5D MK III is better. I have no issue with low light AF speed and the 5D MK III.
We just do not know about the 6D yet, but Canon has cut some features to justify the lower price. I think you will be very happy with a 5D MK II. As they begin selling out, the price will rise, much as it did with the original 5d a few years ago.