Consider using a fluid layout that adapts to the user's screen. Most sites with a fixed layout force the majority of users to view the site targeted to the least common denominator even though 90% of the visitors have a much higher resolution available. This results in layouts that are overly populated with navigational chrome and little content.
If you must use a fixed layout, consider taking a cue from MSN where you split the screen into 760 and 224 pixel columns. If the visitor has a resolution of 800 (which you can detect in JavaScript) then hide the 224 pixel column.
UPDATE from comments: As for determining a safe min though I'd set your screen to 800x600 then browse some of the popular general public sites - MSN, Yahoo, etc. and see what they do. It's a good bet they've invested a lot of research in this area and adopting what they've done is usually a safe bet
