I've read about the new Toshiba XD-E500 being favorably compared to Oppo players. What about my Panasonic S77? I'm satisfied with my current player but the XD-E500 has such a low price I would consider a change. Would I get a big improvement?

If you can purchase it locally...give it a shot. I have the E500 and like the standard def PQ better than that of the XA2. It is somewhat of a matter of taste and screen size. I enjoy sharpness and on my 50" plasmas...I can see no bad effects of its implementation.
I tend not to watch 480i material on my 92" screen...just the 50's.