Statistics still rate 800x600 sites at 27% here. If you want to exclude 27% of potential visitors then do so at your own risk.

I know certain sites cater for computer literate people as their main source of visitors..ie tech sites, but if you have a portfolio site, business site or anything the general populace visits then you will lose potential clients/customers.

It is best to design liquid (fluid) layouts or restrict any fixed size layouts to 750 pixels wide. This will cater for the broadest spectrum of users.

Remember accessibility is making the web available to everyone and not just the elite.