Depends on the page, how much content there is, etc. Most of those silly blog pages can just be made for 800-friendly since the actual content is almost never wider than 700+ pixels and just scroll down forever. In general, if you make a liquid page, you can accommodate most sizes : ) Look at this Digital Point page in different browser sizes... it always fits.
1024 is, by far, the most used today. However, 1 in 10 still use 800 which isn't a small number. It depends on what your target is. For example, if it's a tech savvy target audience than 1024 would be your best bet. I generally still design layouts with a width of 760. But plan on moving up to 980 soon.
A liquid layout is basically a layout that expands to the current width of the user's browser. Digitalforums is a liquid page.
it's good to use 800px below. sometimes users has a resolution of 800x600, its good to be compatible the width size in any resolutions
I think there is no best size. If you have 2 columns then smaller sizes will be better. I prefer creating fixed size sites as they will look fine on all resolutions. Anyway it depends on your project.
Though despite the winding-down popularity of 800x600, I still use it because of the people that do not like to use any other resolution. Whether its because an old monitor/computer that does not support anything better, or someone who prefers that size due to complications with their eye sight, I try to stick to it. (whether I like to or not). For now, because of that, I use 770-780 width on my sites, other than the ones that I try to use a liquid layout for.