If your upgrading from a canon think about the lenses you have they wont fit the Nikon I use the 400D with what i think are great results, mind you the lenses cost more than the camera. Im also looking to upgrade but my next will be the 1D MK3 Canon
Steve
...What he said...
On the other hand I did the opposite and went from Nikon to Canon, so after you've spent time getting to grips with photography, your tastes might have changed so you may well prefer Nikon. I hated my old D40x after literally a month of having it, so when I got my last uni loan I blew it on a Canon 40d and a new lens. (Before anyone starts on that, I do photography at uni so the loan went on necessary equipment)
Tbh mate, you won't notice THAT much of a difference if you're upgrading bodies and sticking with kit lenses, splash out on new glass first, then buy a new body. Although if you can afford to do both liek I did, do it.
i got my 40d body only for £480 off ebay, then my sigma 17-70mm for £250 brand new.... although if I got the lens off ebay I could have saved more money but I was eager to get using the 40d, so didn't want to wait.
Hunt ebay, you'll find some cracking deals.