Admittedly, really low light shooting with little contrast is difficult for the e-620. And the focus assist light isn't well integrated -- it's essentially a strobe from the internal flash - which can be a problem for some people (i.e. photo subjects). I haven't had banding issues, but yes, noise will set in. To be honest, I do not shoot much in low light to begin with, and when I do, the kinds of results I get from the e-620 are fine for me. But I am also a fan of grain effect anyway -- so I may not be the best judge for this as I am less worried/concerned about noise. Oh for the days when I shot film, and felt good about getting what I wanted in focus -- before the motor-car, that was.
The notion of looking at Sony is actually an option I didn't even think about before, but do suggest that you consider still. They have great high-speed shooting capabilities and the see-thru mirror thingie seems to be quite popular in the field with Sony-ites.