[sisforsurfing]

For Nikon I'd look at used D70's and D50's. Look at classifieds sections of sites like nikonians.com for going rates. The D40 is a good camera too but I don't like that it needs AF-S lenses in order to autofocus. I may also sell my D70, and I think there is a D50 for sale on here if I remember right.
Canons are also great and have a great lens selection. I prefer the ergonomics of my Nikon, but as everyone says, go hold them each of them in your hand for a little while. A used 20D is a good buy and a great camera. Usually these can be found packaged with a lens. The new 450D is definitely cool for it's price (same sensor as the 40D except for some image processing?) but I still prefer used prosumer over new consumer. Either way, none of them will make your pictures better instantly and you'll need a learning curve and knowledge of what you're working with. Good luck.