I went from an Oly 3040 to a 5050 and got 66% more pixels, TTL, and RAW (good but slow). This was a big step and I noticed the difference in my pictures.
Then I went from the 5050 to a D70 and got 20% more pixels, no TTL (but soon), no shutter lag, through the lens composition, fast RAW, and interchangeable lenses.
I get more keepers due to better composition and fast RAW. But the picture quality is not a lot better.
Now I ask myself, what will it take to pry my fingers off of my D70?
I have no specific answer. But in general it will have to be a Nikon to amortize my investment in lenses. And the performance will have to be twice as good as I have now ... maybe with a chip with 12mp with as good or better dynamic performance. And I don't think I should have to pay more than I did for my D70 i.e. about $1000.
Until then, I intend to keep my brain focused on getting the most I can out of what I have because the fact is that on most days I'm limited as much by the water I shoot in as I am by the equipment I am using.