D80 vs 450D? I have a preference towards the 450D because of the live view, but apart from that how do they compare?
In a head-to-head?
Resolution: Canon (12.2MP v 10.2MP)
AF System: Nikon (11 points v 9 points and more customisable)
Drive mode: Canon (3.5fps v 3fps)
Viewfinder: Nikon (Pentaprism vs Pentamirror)
LCD Screen: Canon (3" v 2.5")
Noise: Nikon (Just)
Unsurprisingly, it's pretty much a dead heat! Realistically though there are a few differences that are worth considering.
First is the viewfinder. The difference between a proper prism finder (like in the D80) and a mirror (like the 450D) is enormous and it makes a huge difference to the useability of the cameras, especially in lower light conditions. In fact, it's one of the things that makes the "enthusiast" level cameras like the 40D/50D/60D and D80/D90/D7000 cost that much more. However, if you've no experience of either, you won't miss a prism. The main criticism that you see levelled at mirror finders is from people with experience of prisms on older 35mm film cameras.
In short, this is something that you kind of only appreciate once you've used a mirror finder for a while! Given your level of experience and knowledge, I don't think it's an argument winner.
Secondly is the screen. Bigger really is better. They're both pretty low resolution by current standards and, for me, that puts more emphasis on having the biggest screen you can have.
The other real difference is in the AF systems. The Nikon one is more advanced but thats partly because Nikon AF systems have always been "more" in general. More points, more customisable, more flexible. But the Canon systems work; they're accurate and fast. It all comes down to how much you're likely to rely on the AF systems fancy tricks.
I've got a Nikon D90 and it's got a very clever AF system which does 3D tracking using colour information, multi-point wrap around, auto selections and everything else. Yet it spends most of it's time sat in Single Shot (Canon)/AF-S (Nikon) with just the central focus point selected as do many peoples.
If you're likely to do lots of action photography where tracking movement is important (for example where the movement of the subject is unpredictable) then the Nikon wins. If you know the path or route of the subject, or action isn't top priority then it's another draw.
Finally, the ergonomics and handling. The D80 has a top plate LCD and the mode dial on the left of the viewfinder (as you hold the camera). You don't get top plate LCDs on entry level cameras like the 450D and to many, having the mode dial over there on the left is like a badge of honour. A kind of "I saw you looking at my camera and, no, it's not an entry level one - I'm a real photographer".
As ridiculous as this sounds, it does happen. But there are other differences - the Nikon is larger, heavier and more solidly built. I'm sure it's been mentioned before, but the ergonomics make a real difference. It's no joke to say that some cameras just don't fit in some peoples hands. Canon often win here - their ergonomics are often more acceptable to everyone than other companies.
There isn't a right and wrong answer here but I think (and this is my opinion, nothing more) that the Canon is a better match for you. It's slightly more user friendly, it's no less powerful than a D80 and the technology in the 450D is considerably newer than that in the D80.