I use an EF 17-40 f4 L series lens. Itís the cheapest L series lens you can buy, itís very light, but itís optically brilliant.
I use mine on an EOS 50D, so it will go on your 450 comfortably. The thing with the L is it is has a minimum aperture of f4 all the way through the focal range so you donít lose the light capability normally associated with zoom lenses.
Price wise, I think itís cheaper than the 15-85 mentioned above. If you want to stick with ef-s look at the 17-55 f2.8, another awesome lens, but no longer available new, so itíll be ebay for that.
While the 17-40 and 17-55 are undoubtedly fine for landscapes (some may want wider, but these should be OK) are they long enough or fast enough for portrait use?
Probably not, no. I started taking portraits recently with an aperture of f5.6 rather than wide open at f2.8 on my lens because I wanted increased focus on facial features so f4 would be fine. However, for the most part I think I'd prefer a faster lens for portraits and a longer focal length than 40mm too. It is a compromise though having one lens for both.