http://www.upsdell.com/BrowserNews/stat_trends.htm
800x600: ~30% of page accesses.
1024x768 and higher: ~67% of page accesses
The best approach is to design pages which size themselves, because anything else means you are not optimized for a sizeable chunk of your audience. However that can be fairly tricky with graphic-intensive sites, so for now if you work on fixed sizes, you pretty much have to design for the smaller size.
Which seems to be the general practise. What we don't seem to have as good a grasp of (judging by the tendency towards massive page sizes) is that even in the US broadband penetration is still only around 55%. Korea is among a handful of countries with higher broadband penetration, but otherwise the rest of the world isn't even at US levels.
On the topic of compatibility, it's also interesting that the use of IE has fallen from a high of 94% down to around 84% and still declining. That's relevant to anyone using CSS because Explorer has a lot of non-compliancy issues. If you checked your design in IE, there is a good chance your site won't look right in Netscape, Firefox, Opera or whatever, unless you included some workarounds.