If you bought your computer from a company like dell, hp, gateway, etc. you should be able to find what type of ram is installed in it somewhere in the owner's manual. It might also be somewhere on the stick. Otherwise you might need to take it to a computer store for them to tell you what kind it is. It should be like sdram or rdram and either 168 or 184 pins, unless it's pretty new in which case you may have ddr2 memory. After that check at a store like newegg.com and find the memory section. Then under type, select what kind it is and select whatever other search options you want (like size). I'd probably shoot for a 1gb stick if you want to spend that much money.