The subject of designing for screen resolutions has always been a point of contention, but the trend these days does seem to be designing web sites for 1024×768 and above, even with fluid layout sites. It’s almost de-facto and rarely questioned. But is it right?
Here at IPC we’ve always gone with a compromise - go wider, but still design for 800×600 in a way. The trick is to keep the core content within about 780px and have a few ‘non essential’ elements hanging off on the right - see NME.com as an example. It’s a balance between getting more space (and more ads!) above the fold, and catering for the lowest common denominator - my gut feeling has always been they’re not such a minority.

So I thought i’d test my gut feeling and look at the stats for a range of our sites (Horse and Hound, Web User, NME, Wallpaper, Whats On TV, and Nuts), and here are the results:
Only 12.5% of visitors have an 800×600 screen resolution - BUT 21.5% of visitors have an actual browser width of 800px or less.
My theory has been that a surprisingly large number of people surf with their sidebars open - eg their bookmarks or history - my Dad included. I would guesstimate this to be as high as 50% across the board.
So despite the vast majority now having a decent resolutionof 1024px and above (640×480 is practically zero these days, thank God), a size-able proportion - higher than FireFox’s market share - won’t appreciate any content being out wider than 800px. For example, the redesigned telegraph.co.uk homepage:

As with most things on t’interweb, it all comes down to graceful degradation, or even better progressive enhancement. Bare it in mind please!