As larger screens become cheaper so more people will be using them. But the majority of users will not have upgraded to the most modern equipment, especially considering the economic climate world wide.

1024 x 768 is now a standard that should be considered for fix sized web pages. 800 x 600 admittedly will allow accessibility to the now very small minority that use this definition. However depending on who you are targeting with your site, you give these people a priority. That priority would be minimal, if you are running a commercial website selling ocean going yachts and slightly larger if you have a community information site for pensioners.

If you are aiming your site at techies, then perhaps you should consider using higher resolutions or javascript to access your users resolution and multiple css to change your layout accordingly. This is lots of hassle but if your clients are willing to pay for it .. go go go!

Realistically until monitor resolutions become much higher definition, perhaps similar to that of good quality printed page, I would not look to change from 1024 x 768. Those people who are trawling the web with these higher resolutions are not going to enjoy the experience. They should learn what resolution most pages are written for and stick to it. Unless of course they are trying to make a pedantic point about something they know little about. As it would seem your clients did.

Take them in some logs from one of your well used sites, with screen resolutions on, and then, my own comments would be something along the lines of "I understand what you are saying but, no one else really uses high screen resolutions yet. In 5 years time we can certainly look at these figures again and update as necessary."

In the mean time use fixed width pages using relative values for borders padding and font sizes in CSS to allow resizing of text in browsers. This solves the accessibility issue. Changing your pages to suit people with very high resolutions should not be done yet, unless your clients of course want it and are prepared to pay handsomely for it.