I've always heard that 800x600 is more of a courtesy for people who don't browse in maximised windows.

I use a 1440 x 900 resolution but I am currently browsing in a 1300x720 window (just the website not the nav bar or anything). However I change my browser size (width especially) quite frequently depending on what other programs I have up and which sites I'm viewing.

My biggest issue when I was making a website was Mac vs. PC. I had the whole thing absolutely fantastic looking on both Safari and Firefox on my Mac, but when I switched over to my PC, IE and Firefox had their own issues. I was running up and down stairs to switch between computers for a long time. Who says there's no such thing as geeks exercising?

Good luck.