Nikon D80 or Canon 400D
Both are capable of taking outstanding photos. Both are compatible with a dizzying array of lenses, ranging from (comparatively) low cost consumer lenses to pro-level lenses costing thou$and$.
Not sure if you've had an opportunity to handle and look through the cameras you're considering ? I strongly recommend you do that, if at all possible. Comparing specs and reading others' opinions can only take you so far.
When you do visit the shop, I'd suggest you handle each camera with the lens(es) you're likely to have for starters, rather than whatever the store has mounted on the camera when you arrive. One camera may feel much better than the other, taking weight, shape, and ergonomics into account.
The Nikon, which costs more, has the much better viewfinder in my opinion. That would be a very important factor for me.