Upgrading RAM

Back in March, I posted about my computer being slow and a couple of people here were very helpful and told me to upgrade my RAM. Now that I'm in school and using the laptop more, I definitely need to do this.  But I have no idea how to be sure I'm buying the right thing.  I can find 2gb sticks on Amazon that say they're for Inspiron 1501, but they don't have technical specs so that I can match them to the RAM from Dell.  

Any suggestions on where to buy RAM for this laptop?  Thanks!