Just read your introduction in the Welcome thread so I,too, welcome you.

I have the D80 and am quite pleased with it. D90 does a far better job handling noise at high iso but then it will cost you a lot more.
I was going to say stay with Nikon because you already have a lens but if that lens is shot, then I guess it could go either way. I'm prone to Nikon but I'm a prejudiced owner.

Either way, you are marrying into a system. Sure they make adapters so you can mount a Nikon lens on a Canon and vice-versa but you lose a lot that way.
So check out the systems and don't forget Olympus, Pentax, Sony.
They make good cameras too and are more affordable.

I do want you to read the camera dealer feedback forum. Good deals are a dime a dozen, good dealers aren't.
When you shop ebay, make sure the dealer for whichever brand you end up with is an
AUTHORIZED dealer. If they aren't, then they are selling grey market cameras that were meant to be sold in other countries. Nikon and I guess Canon will not honor their warranties if they are gray market. The dealer will offer a warranty but it won't be from the manufacturer and I've heard many horror stories of the so called warranty companies.

Good luck and stick around. Many here with more knowledge than I, so ask all the questions you need.