There's a question of which camera differences are significant to you. If a feature isn't important to you, it's probably not worth paying for. I see two things that the Nikon D90 (and D300) can do that cheaper Nikon cameras cannot do that would be significant to my purchase decision. You may have other criteria.
The Nikon D50/D70/D80/D90/D200/D300 cameras can auto-focus with AF lenses that lack built-in motors, but the D40/D60/D3000/D5000 cameras cannot. Eventually, this distinction will matter less as Nikon continues to very slowly convert its lens lineup to AF-S
. Nikon has been working on the transition to internal motors since 1992. They now have 30 AF-S lenses available with 21 currently-produced
AF lenses left to replace. The old AF lenses will remain in the used market for a few decades after that. With this year's availability of 35mm and 50mm AF-S prime lenses, you can now buy most of the lenses that you might want in AF-S. The AF lenses are mostly just a less expensive alternative.
The CMOS sensors in the Nikon D90 (and D300) have much better high ISO performance than the CCD sensors in the D80 (and D200) models that they replaced. The just-announced D5000 also uses a CMOS sensor with good high ISO performance, while the new D3000 uses the same old CCD sensor as the D60. Eventually, improved sensor technology will make its way down to Nikon's low-end camera models.
In the Canon line, those two sharp distinctions don't exist between camera models. The differences in high-ISO performance between different generations of Canon CMOS sensors aren't as dramatic. All Canon EF and EF-S lenses
produced since 1987 are fully-functional with even the cheapest Canon EOS DSLR. You will be looking more at other items like the size of the camera body, the top display, wheels and knobs, auto-focus system, viewfinder, resolution, video capability, shooting speed, etc. to determine which Canon EOS camera model might suit your needs.