I recently purchased an Oppo 970 to feed my Gennum VXP-based Anthem AVM50 480i via HDMI. The picture is noticeably smoother/more artifact-free and with less noise to my eyes than either 480p or 1080p from the XA2. It's also noticeably better than feeding 480i over component (less artifacting and noise) from either the XA2 or my older Sony DVP9000. The jump in PQ quality over 480p from the XA2 was significant for me on a 110 inch screen (Sony Pearl pj) and I'd cautiously say even just short of dramatic
This confuses me somewhat considering the contrasting results of the OP feeding 480i via SDI to the Gennum-based Crystallio II. IMO the Gennum-based AVM50 handily outperforms the XA2 for SDDVD sources. SDI should have provided still yet another step up in quality.
As has been mentioned, scaling is only part of the equation. For completeness' sake, however, here are all three pics of the AVIA resolution pattern. Be sure to maximize them for clarity.