Not too long ago websites used to be designed to fit onto the screen using 800x600 resolution.
These days it seems more common that sites fit onto a 1024x768 resolution.
Have you checked your stats on what screen resolutions your visitors are using?
I used Google Analytics, and it seems that 800x600 users are now a VERY small percentage, with the far majority being 1024x768 and over.
It'll be interesting to see how new designs continue to change.