What Width

Hello All

I'm just going to design a website for myself. I was wondering what width I can use that would look good on the majority of screens. For example, all sites I go to, from the BBC to Tesco, etc look good even if I'm on my widescreen laptop, my work PC or a standard 17" monitor, etc. How is this achieved?

Although I've had X5 and X6 since release I've only really just got round to designing something.

Any help would be much appreciated.

Thanks