1. If it is a desktop PC usually it will have DIMM RAM; If it is a Laptop it will use SO-DIMM RAM
2. You have to choose the correct RAM, the frequency the type, e.g PC2-5300 667MHZ Or PC3200 400MHZ. To check out what your computer is compatible download Everest Home Edition (http://www.lavalys.com/) and it will do a check on your computer and give you information about the currently installed RAM. Alternative, you can just simply go to Crucial (http://www.crucial.com/) or Corsair (http://www.corsair.com/) websites, and select your computer model to check for your compatible RAM.
3. After you have found out what RAM to buy, go ahead and buy one from eBay. You would want to buy a good reliable brand such as Kingston, Corsair etc, to obtain stability in your system. It would just cost around $60-$70 for a 2GB Kingston or Corsair Ram nowadays.
4. Now just enjoy the high speed of your computer after install the correct RAM...:)