given a perfect scenario, i think good or gooder or goodest skills with shooting or postprocessing can help you get away with unfavourable image quality (eg. noise, colours that you don't like, sharpness etc...)
but here are some factors that I think a user will never overcome regardless of photography skill levels
1) buttons, grips, handling, ergonomics (maybe you might get used to it, but maybe you wont too)
2) viewfinder (dont underestimate this. The D80 has an awesome viewfinder, haven't seen the 450d's one though)
3) weight & size, factoring in the body and lens available (even if you get used to it, 1kg is still 1kg. time to build some biceps
)
4) the DSLR system, technical support, servicing standards blah blah blah ... (every brand will keep improving, but somehow some will manage to stay ahead of others in some areas. be it price, availability, quality control, variety of products, 3rd party availability etc... For example, at this point of time, I just can't find a cheap 1st hand prime lens for Sony, and likewise I just can't find an image stabilized 50mm f/1.4 lens for Canon. find out which products you think will be important to yourself)
5) community (not really important, but it makes shooting more fun!! in my opinion, 2nd market is really important if you're a gearhead)
6) $$$ (ultimately, this is the MOST important hahahha :b)
Features and technical specs change/improve so fast your latest camera becomes obsolete soon. So try to let your photography prove that you're worth more than your camera.
Pick something you feel comfortable with in your hands, and a system that you believe you can grow with if you want to constantly upgrade or accessorize.
Both Nikon and Canon have an awesome DSLR system and community, you can't go wrong with either brands. That being said, so does Pentax, Olympus and Sony too :b
So with regards to your question, I'd base my decision on whether the ergonomics/viewfinder (of the D80) is more important to me, or the features and compactness (of the 450d) is more important to me.
I hope you make your decision well =)
cheers!