Well, since web design has been my full time job for the last 6 years, I can tell you if at all possible, don't go above 800x600 for a site design. A vast majority of people are at that rez, and there isn't much reason to build a site that requires any more room than that.
I run at 1024x768 at home (considerably larger at work) but rarely have the browser maxed full screen, so you can't always go just by screen rez either. I usually run my browser at 900x700 or so, regardless of the resoltion my monitor is.
As far as making a "liquid" design, just stick w/ percentages for widths on stuff. For the beginner, that should allow you to keep things consistant between different resolutions pretty simply.