=if($a1<=20,"Low",if($a1<=50,"Medium",""))
This formula assumes that A1 is always greater than 0, otherwise it gets more complicated, let me know if thats the case though. Also, inside the "", you can place text if none of the conditions are met.
As you can see it can get a little messy to read, so you could name the table column and reference the column in the formula or if its not part of a table name the particular cell.
In Excel 2007, it's easier to refer to columns using [] without the need to name them (this is a lot easier to maintain and read should you need to change column references in the future).