Photoshop Mock Up

New File at designated width you designed for (accounting for the most popular screen resolution + browser + browser settings)
1024 x 768 ie6 is 1003 × 568
800 x 600 ie6 is 779 × 400
If I don't have a lot of content, I might as well design to stay safe at 779 x 400.

I could standardize this part of the process by always creating a file 1024 x 1000 px
Left justified: place vertical markers at 779px & 1003px and horizontal markers at 400px & 568px
Centered: place vertical markers at 10px, 112px, 902px, & 1014 and horizontal markers at 400px & 568px

Then I would place the most important content in the main 779 x 400 area and have extra room to build into and crop when I'm done