You can switch from a drop down menu to a popup menu and it will only display the second value.
Otherwise, you can overlap your numeric key field, with another field from the related record, showing the related value that you want.
This way the field behind the display field is used for selection (enable the arrow so people can select it), but the display field is always on top.
Ah I love the first option! That's exactly what I was looking for. Thanks!