Some say 770 for old school 800x600 monitors, some say 1000 for 1024x768, my monitor's current resolution is 1920x1200, so what's the right width to use? Really it's arbitrary to what your wanting to achieve and who the user is. I hear alot of people talk about the magic number 960 as if it holds the key but really it's just the most divisible for CSS type layouts with its 12-16 column range. And then there is those pesky full screen websites in Flash that break all the rules. So what's right, nothing really. Go out there and look at sites and they come in all width sizes, some are 800 some are 900 some are 990 and some are even 1000. So if it's 1000 and the monitor is set to 1024, that should avoid the horizontal scroll right, wrong. What about the vertical scroll bar, that of course changes sizes from a Mac to a PC and from IE to Safari. So, I've scoured both the window browsers and mac browsers and below is an example of what I mean: 

So from now on I think the range from 960-990 would be a better margin for avoiding that terrible horizontal scroller.