Suspect: I agree that for all your requirements all you need is an upgrade - though I'd go for 450D and up for the better viewfinder. This should sort you out for everything except high ISO noise - though some report that the 500D is good for that too, though I doubt as good as the Nikons you mention. Other reports suggest no better than 450D which is good to around 400 or 800. If you wanted to stick with Canon then a cheap(ish) 2nd hand 5D produces very clean images at 1600. Amazing for a camera around 4 years on, but they're heavy and bulky, as are the lenses that do it justice.
I also agree to a certain extent with you about the MP war. For amateur use I'd be really happy with around 8MP (maybe even less) but both Chris and I need to consider requirements for editorial use. (8MP on an APSc size modern sensor should be very quiet these days too at high ISOs). For a double page spread (DPS) at the industry standard of 300dpi you need around 18MP, so even the 15MP 500D doesnt quite cut it without upsizing. For home printing though I'd be happy at 200dpi, and most prints would be A4 or less, thus reducing MP requirements.
"15MP doesn't translate into better image quality" :
this depends on the processor. They're getting better at dealing with noise. If files are clean, the higher resolution should produce a more detailed image. The E-P1 produces good image quality on a smallish m4/3s sensor. Why - because of the processor. About a year ago I thought we were coming to the limits of how many MP we could squeeze out of a sensor, but it seems image quality remains good with ever more MPs. This is good for Pro use, but as you say, we've aleady gone beyond what the average amateur needs, and for most purposes the large files are a disadvantage.