100%

For pixels, I mean. I thought I read somewhere that it's better (speedwise) to define pixels then percentage.

Keyplr means that if you want a page using maximum width for both 800x600 AND 1024*768, then he is correct, you should set the width to 100% :)

Nonetheless, for a maximum width in pixel for 800x600 AND 1024x768 RESPECTIVELY, I usually remove 20px from the width (that is 780 px and 1004 respectively). Not the professional way, but it works. May someone else can post the exact width.

I use the 20px system as well when I need to write fixed-width pages.

However, 100% is the proper way to write it so it flows regardless of resolution.

An issue on that though - there is no "proper" resolution. Even though I work on a 21" monitor, I don't always use my browser full-screen. SO, if you have an image that will be 21" long, I might not see it all anyway.

You can't avoid scroll bars, period.

The only items I use pixel dimensions for are images, I hate seeing the page jump when the browser sees the actual size of the image and shifts everything for it to fit.
Page widths, tables (yes, I'm still using tables for layout) and any other layout stuff is done using percentages (I'm beginning to use ems for font sizes).
This gives me a fair facsimily of a fluid site.