NOTE: You do not need to buy the RAM at Kingston. They just have a great tool to find out you options.
1. Go to kingston.com
2. Click "Product info" in the upper left
3. Click "SEARCH BY MFGR" near the top of the left column
4. Change the first search box to Dell
5. Enter the model number of the Dell in the second box
6. Click search
7. Find your PC in the list
It should tell you what type of memory, what size and how much you can put in your computer. Also if it needs to be in pairs, etc. Mom and Pop computer stores usually have good RAM prices.