eyeye: What do you have for lenses now? I'm surprised no one has asked this yet, as it's a pretty important deciding factor.
The 5D (mkI) is going to give you what you want, which is clean photos at ISO400 & 800, and usable photos at ISO1600. The mkII will give you clean photos all the way up to 3200, and usable photos at 6400.
So, if you don't have nice lenses now (which I don't know, because no one asked), the 5D is an excellent camera. If your lenses are good, then you might as well get the mkII.