Well from a design aspect, its not always easy to go with fluid width, so I always go between 900-1000. Small enough for 1024x768, but slightly too big for 800x600. But I figure, if you're still on 800x600, you probably don't care anyway.
I use 800 - 1000 usually. I am on 1280 x 1024, and hopefully nobody is on 800-600 anymore so 800 should be fine for most people.
As I said on SitePoint, I let the site's width be governed (within reason) by the visitor's viewport. At a minimum, I'll set it for 750px - just because a visitor's screen resolution is set to 1024x768 does not mean the browser will be maximized to fill all that space.
Width is actually very difficult one - whilst the vast majority of PC users now have 1024 or larger and most will have their browser maximised there is a new consideration which is the ever increasing volume of hand held devices such as PDAs and Blackberrys that are being used to access the internet. For sites which are unlikely to be used on these types of devices we typically use a 800px-950px width. For those that are it will vary from using propriety designs/ applications for them or not. From a usability point of view you should ideally use a dynamic width (not done by javascript) to enable the page to scale to the size of the current window with a maximum width set for those that have very high resolution screens so they dont have whole paragraphs appearing as a single stupidly long line. Reallity is that this is exceptionally difficult to do well and typically is too constraining on design when you also consider that text size should be relative/ dynamic rather than fixed too and so a fixed width is the easier path to take.