That's awesome Steve. :thumbsup:
If I were in your shoes, my preference would be to exchange for either the Canon or Nikon, for the simple reason is that they are, generally speaking, the best. I was going to say the 400D has live view, but I think I'm mistaking that for the 40D, sorry.
Back to my Nikon/Canon theory, one of the key points of SLRs is changing lenses to suit your needs. You're never going to struggle to find someone that stocks a good range of Canon or Nikon lenses. They're the main brands and help and advice with either should be plentiful, either online or in stores.
Also,
check this out. I also recommend going into stores and trying out the cameras for yourself. You're never going to get a perfect feel for cameras in a relatively short timeframe, but you'll get a better idea than just listening to us!
I've been very happy with my 350D which has almost 11,000 frames taken on it now. A friend just bought a Nikon D80 and it looks great as well, and its gridlines on demand for the viewfinder is something I miss from my old Nikon F80 35mm SLR. The Canon might still have the edge for low noise, def the case when I bought in March last year prior to the release of the D80, but I'd be much happier recommending the D80 now, although it does cost a bit more.
Anyway, enjoy your birthday and good luck with whatever you decide with your camera.
Cheers Liam