Easiest way to figure that out for your specific site is to get a free counter (do a search, there's tons) and get one that records other information from the visitor. Many of them grab the screen resolution.
However as a standard, 800 x 600 and 1024 x 768 are pretty close. However every day it migrates more and more towards the larer sizes.
Personally, I design for 1024x768 because that's what most people have been using for years and if soem old fogey with a 5 year old PC and a 14" monitor want to view my ste, why shoudl I make it look crappy for everyone else. Not gonna happen.