Here is the latest resolution data:
http://www.w3schools.com/browsers/browsers_display.asp
As you can see, there are still some 800x600 folks out there, but 1024x768 is by far the most popular resolution. You need to determine for yourself who to target based on your customer's demographics. Sites catering to the elderly for example may want to keep 800x600 optimization, but I have found that targeting 1024x768 is your best bet to maximize selling space and preventing 'empty' space on higher resolution (laptop) computers.