1) Open My Computer
2) On the frame on the left, choose View System Information
3) Click on Advanced
4) Click on the Settings button under Peformance.
5) Click on Advanced.
6) Click on Change.
It should tell you how much is recommended and what is the maximum size you can set. I believe that this varies depending on the size of your hard drive. However, if your motherboard can support more RAM, I highly recommend getting more as it will improve the performance of all your programs, and the performance of Windows itself.
www.chiefvalue.com has really good deals on RAM, and RAM is generally easy to install. You just have to make sure that you discharge any static electricity on you before you try installing it, or you may fry it.