Resolution wise there is very little advantage to an e510/e520 over a e500. However, the IS and (I'll add) VERY clean high iso are nice/worthwhile upgrades from the e500 perspective. Another one for RAW shooters like myself is the 8 consecutive frames before the buffer fills(e510). I tend to fire off 2-3 shots then wait a second or two and then fire off another short burst. My e500 fills up right away, but a 510/520 would suffice for my type of event shooting. The new type of IS for vertical panning is also very useful for me - I only use panning when shooting sports and I use the camera vertically for that. The horizontal IS mode for panning on the e510 would have been useless for me. I also like their decision to include a larger screen (2.7") rather than a smaller flip out one. I don't use the flip out lcd on my 8080 much.
So although my e500 is still terrific for what I need it to do, if I need a 2nd dslr this summer I will have no hesitation in picking up a e520.