I've been having this dilemma of whether to optimize my site for 1280 or 1024 px. (horizontal width).
I didn't find a reliable statistics about monitors's sizes and resolutions. It's all very approx.
I'm thinking that probably 1024 is better because of 2 reasons:
My potential customers live in poorer countries (3rd world, former USSR)where 15'' and 17'' monitors are still the most common ones and the second reason is that overall, if you have to choose between 2 "evils"...I think it's better to have a 1024 px optimized site on 1280 screen (and see some empty space on the sides), than to get a horizontal scroll bar on 1024 monitors if the site were to be optimized for 1280.