I've been making covers in 3 sizes for my ebooks: 500x700 pixel for the covers embedded into the books themselves; 150x210 pixel "large thumbnails" for my main webpage displays, and 100x140 pixel "small thumbnails" for multiple-novel pages. (See my site for examples of all.) I hadn't heard about Amazon's new size limitations, but these sizes always worked well for the ebooks and other display needs.
Your 600x800 covers may not have shown up if you saved them in the wrong file format. There is a slight difference in saving files for the web that compresses them differently than standard JPG and GIF files (in Photoshop, you need to select "Save for Web and Devices" in the File menu); many ebook readers will only handle files saved for web formats, even if the size is the same.