I've always kept with 960px
as it's viewable on 1024x768
, and is cleanly divisible by 2, 3, 4, 5, 6, 8, 10, 12, 15, 16, 20, 24, 30, 32, 40, 48, 60, 64, 80, 96, 120, 160, 192, 240, 320 and 480... so I can split up the design of my site (using the rule of thirds) cleanly.
EDIT after a few years:
Please consider using CSS Media Queries to make your content responsive to the myriad of devices out there now if you can't use a fluid layout. I'm not sure that one can claim one pixel width to rule them all in 2012.