I doubt that there are more than 1% of people still using this size monitor, so forget about them.
800 pixels is an oft-quoted figure for optimum display, but in truth you could go wider. A very large number of visitors will be using 1024x768 display sizes and above. If you want to be very safe however, you should aim for a pixel width of about 760. That's because your pages may well go beyond 600 pixels in height as you add content, and browsers will insert a scroll bar which will obscure around 30 or 40 pixels at the right of the screen.