I'll take a slightly different tack on this. It depends on your audience.
If you expect tech savvy people to be the real users, use 600 x 800 since it will fit nicely with 1024 screens even with the browser headers, etc.
However, if you want to make it easy for everyone, remember that a lot of people out there run the screens at 600 x 800, don't know how to resize a header and never close the "Favorites" list. So try to keep in within 500 x 700 or so. I figured this out after posting a lot of photos at 600 x 800 and having everyone complain they couldn't see the whole picture at once.