Selecting TOP 20 Products Per Catagory

Hi

I'm trying to select the TOP 20 Products per Catagory out of my Star Topology Database
if someone would PLEASE be able to help me out.

Here is my Query: -

SELECT Distinct (SELECT TOP 20 DP.[Description]), DP.Mims_Sub_Cat, SUM(FD.Cost) AS 'Cost'
FROM DIM_Product DP, FACT_Dispensary FD, DIM_Time DT, DIM_Client DC
WHERE DP.Product_KEY = FD.Product_Key
AND FD.Time_KEY = DT.Time_KEY
AND FD.Client_Key = DC.Client_KEY
AND DT.[Year] = 2007
AND DT.[Month] IN (2)
AND Client_name LIKE '%Medicare%'
AND DP.Manufacture_Name LIKE '%Cipla%'
--AND DP.[Description] IN (SELECT TOP 20 [Description])
AND DP.Mims_Sub_Cat IN (SELECT Mims_Sub_Cat)
GROUP BY DP.Mims_Sub_Cat, DP.[Description], FD.Cost
ORDER by DP.Mims_Sub_Cat, SUM(FD.Cost) DESC

My other problem is that it keeps on selecting the same products although i have a distinct in my query
The query will select a product with the amount and then select the same product again with a different amount.

Example :-

Product Descripion Catagory Cost
ULTAK TABS 150MG 60 Acid Reducers 384.94
ULTAK TABS 150MG 60 Acid Reducers 85.54
ULTAK TABS 150MG 60 Acid Reducers 56.98