Excel 2003 -- Conditonal formatting more than 3

i could really use some help with this one!
I am trying to create a spreadsheet (excel version 2003) using conditional formatting. This will be regarding changing the colour of the cell if a student's grade changes. I know there is a way of having more than 3 colour options. I've had a go with some visual basic, but it only works with numbers and not letters .I need the colour to change as the letter changes. Below is an example of the code i have used to change if numbers are used;

Private Sub Worksheet_Change(ByVal Target As Range)
Set I = Intersect(Target, Range("A2:B8"))
If Not I Is Nothing Then
Select Case Target
Case 0 To 100: NewColor = 37 ' light blue

Case 101 To 200: NewColor = 46 ' orange

This works fine with numbers. But have no idea what to do when it come to using letters
Thanks in advance! :-)