I'm not really sure why the XTi isn't OK for what you are doing, but if you want to upgrade you have to decide what you want in the new camera.
The
Canon Digital Rebel T2i would be your lowest cost option, It gives you HD video, a higher resolution sensor and a number of other upgrade features.
The
Canon EOS 60D gives you a tilt and swivel LCD screen, slightly better autofocus, a faster frame rate, a larger viewfinder and an extra rear control dial to make using the camera slightly easier (and quicker).
The
Canon EOS 7D gives you just about everything you could want. High speed operation (8 frames/sec), better autofocus, HD video, electronic level indicators etc.
If you wanted the ultimate in image quality then a full frame camera like the
Canon EOS 5D MkII is probably what you'd want to look at.