Can't tell you how my E-510 compares to the E-500. But the key difference, in my opinion, is image stabilization. Because of your concern with slow shutter speeds that causes blur, IS is important.
As for 510 vs 520 image quality. I still think the 520 has less noise. But the noise reduction seems to take away from the sharpness making images look softer than the 510. I guess this whole compare and contrast is moot though if you can't a hold of a 510 in your area. If you don't mind purchasing online, there are vendors that still sell thay 510.
I cant believe that my last post did not go through. As a side not I am having lots of issues with this forum's database. I am getting the php overload message every 3rd time when I poke around.
This time it looks like that I lost the last message.
I asked if I could take out that noise processing, and do it instead in the software - only if I find it critical ? I am tolerent to the noise much more then to the lack of sharpness and blurrines. I guess that is a personal preference.
As for not buying over the internet, lets say I am not inclined due to past experience. I also like that instant feel of reward when I walk in the store and then walk out with the new toy. Much better then waiting for shipping, worrying about damage in the transport, shipping back, and then waiting for new period of time. I guess thats why you pay the premium.