5D II for concerts?

Does anyone here use the 5D (mk I or II) for concert photography?

I currently own a 450D, but I am wondering if I should upgrade to the 5D II, as I might be getting a litte more serious with my photography.

I like my 450D, but there are a few things I hope would be better if I upgraded. I assume the handling of the camera is better, with more buttons for quick access to settings. The viewfinder of course is a plus.

But I am wondering how many usable stops of ISO I gain. I´m happy with my 450D up to ISO 800. 1600 is OK. But in some situasions I need to go even higher, as the shutter speed gets too slow.

I shoot mostly rock concerts, and occasionally intimate acoustic gigs, with very dim lighting.

I am a little concerned about the autofocus. I´ve read that people fint it difficult to focus in poor light. But I´m doing fine with my 450D... It´s not worse is it?

Any other thoughts about whether this is the right camera for this kind of shooting would be welcomed.