I'd expect the XTi to give you not only more resolution but to also give you better high-ISO performance, but I don't have numbers to back that up. Otherwise I think your comments and the other ideas mentioned in this thread cover it.
The 20D is much better at anything above ISO 400, as far as noise is concerned. The XTi has read noise 1.1 stops higher, relative to signal, than the the 20D at ISO 1600, with the same Tv and Av values and the same subject and lighting.
The shadows are a tad better at ISOs 100 and 200 on the XTi, assuming you expose to the RAW "right" on both cameras.