Use HLOOKUP.

If you have more complex calculations or formulas start slowing down your workbook, then try using an onchange event:

Private Sub Worksheet_Change(ByVal Target As Range)
    If Target.Address = "$A$1" Then
        If Range("A1").Value = "Item1" Then
            ' Calculate Stuff
        ElseIf Range("A1").Value = "Item2" Then
            ' Calculate Stuff
        End If
    End If
End Sub