Don't be so quick to discount brands outside the "Big two". The Olympus line is generally regarded as being very good, and, as mentioned has outstanding, and (relatively) reasonably priced optics. One consideration with the 4/3 format is the crop factor. It's 2:1 vice 1.5:1, so your 50mm lens gives a FOV equivalent to that of a 100mm lens on a full-frame sensor. Great if you're doing tele work, not so much if you're doing wide-angle stuff. I can't comment on the noise aspect, but I suspect some on-line research at 'sites such as DPReview will give you some answers.

Pentax and Sony are both good names as well; Pentax is one of the original 'big names' in the consumer camera world, and their current line up is top-notch. As with Nikon and Canon, there is a vast array of legacy lenses, many of which can be aquired very cheaply, that you can use.

Your best bet is to figure out what your top two, three priorities are (eg low noise, high frame-rate, etc), determine a budget, and head down to your local camera store. Give the sales clerk the info and play with the different models that he shows you; ergonomics are very important, so DON'T buy one that you haven't played with. Once you've narrowed the field down to one or two, go home and spend a few days doing research before finalizing your decision.