It largely depends on the users. The customers we've for our software (an timecard system) often have "hand-me-down" computers (folks in payroll department rarely get the new computers). The other item we've noticed is most of the folks we're dealing with are generally older -- so it's more likley for us to see folks running in "large font" mode. With those factors in mind, our applications are targeted at 800x600.

The other item that becomes of concern is vertical resolution -- a rule of thumb I use is to get the same vertical resolution on a wide-screen monitor as you would get on a 4x3 monitor, you'll need to go one or sizes larger (e.g. to get the same size as a 17" diagonal 4x3 monitor, you need to get a 21" or 23" diagonal wide-screen monitor).