There has been so many debates on website resolution its crazy. Do a Google search and take the time to read some of the articles and blog posts. Both fixed and fluid have their advantages/disadvantages. At this point in time I usually go fixed at ~750x550 to accommodate the users out there still using 800x600 resolution as well.
Fluid on the other hand might be something to think about if you ever plan on having a mobile friendly website for mobile devices
Just my opinion.
