Whever I post images on line, I always size them vertically to 600 pixels wide. The annoying problem of scrolling horizontally will never be my fault!
I think in general, 1024x768 is pretty much the standard resolution for computers out there today. As a web designer, I usually build my sites to look good at that resoultion or higher, but still be viewable at 800x600. I generally disregard people who view at 640x480.
Bart