I need to be able to dynamically set the valid range of selectable values in
a second cell based on the value entered in the first cell. Specifically,
when my users select a six-digit account number from a drop-down box (using
DataValidation), I need the cell to the right to restrict its possible range
of selectable values based on what account number was chosen.

I am using Excel 2003 and I have a good bit of experience using VBA if
that's the tool I should be using.