The Canon vs Nikon debates are irresolvable. The 450D has a lot more features than the D40 which is obvious because of the price difference. It has sensor cleaning, 9 points focus, ISO button, DoF preview, live view (yes, like p&s cameras), bigger LCD screen, white balance button. It also has bracketing, which you'd find very useful for
HDR images. The equivalent of this in the Nikon world is D90, which is quite expensive.
Thing is, these features may not be entirely relevant to you in the beginning. Don't worry about what websites say, they're usually biased, but I can tell you that Nikon cameras have more 'color saturation' in the photos, so the photos appear more vibrant. Canon photos appear more detailed. You won't be disappointed with any of the 3 cameras on your list.
Yes, choosing is tough and I spent about 1.5 months looking and reading.
Have a look through grilkip's, wossname's, penagate'ss, ILMV's and mendhak's photostreams. Grilkip and Penagate use Canons, wossname and mendhak and ILMV use Nikons. I think, between us, we cover several areas of photography (flowers, landscapes, stupid cars, close ups, people).