As mentioned MP counts between 6 and 8 is actually not that critical, unless you want to print anything really big. I've heard people who use 6MP SLR and still get a good result on a 11x20 print. So I think 6MP should be plenty. The additional issue for MP count in Olympus's case is this. E1 is a 5MP camera, for the E300/E500 they bump the MP count for the same sensor to 8MP, which is more taxing for the smaller sensor, and as a result worsen the noise situation for ISO level higher the 400. While I cannot say personally I've seen the picture first hand, most reviews do agree with this point.
KM 5D and Nikon D50/D70 actually shares the same Sony sensor, with 5D actually offers ISO 80-100, which I guess may be a bonus. But I've also seen in some review 5D's noise characteristic may not be as good as the Nikon, though I found no such issue with mine. I've heard Nikon D50 is a very user friendly camera, with built-in help function and such, and it was a tough decision between the D50 and 5D. I went with 5D because it offers the built-in image stablization, uses CF card, and I was a previous Minolta user in my DiMAGE Z1.