Which camera is better Canon EOS 400D or Nikon D80?

Theres a bunch of comparison tests out there for these two models. Check more at http://alatest.com/canon_eos_400d_digital_rebel_xti/digital_slr_cameras/expert_reviews/pe-34087417,248/

Many expert review sources claim the EOS 400D to have better color accuracy and high ISO dynamic range, where as the D80 has better autofocus and build quality.

I would consider what existing equipment you already have and what you aim to do with the camera long term. Both have the capacity to be more than just an entry SLR and especially the D80 can be an appropriate backup body for most prosumers out there.