well using an adapter, you can use Nikon lenses on canon body.
The Tamron is a good lens, its just that when you need to have it serviced,,,may or may not be troublesome.
Over the 450D i recommend instead the 1000D. check it out. However if you are going for the 450D Twin IS kit then is different story.
If your previous SLR camera was Canon, guess what..can reuse the lens just that manual focus only.
as for which lens, try first to see which lens suits you.