I would get the most I could afford to buy. BUT first check your motherboard's manual! Be sure to buy the kind of ram it is able to handle. Also, there is a limit as to how much it can handle. In all probability, it should be able to handle at least 1gb.
Hi, I suggest you goto the website of your computer (i.e. dell, sony, toshiba, etc) type in your pc model number and you can get the maximum amount of ram the pc is manufactured to handle.
If your pc is not a branded one,like dell,hp and so on,then you can check for your motherboard specific model and make a search on internet about its specifications for the memory.If you find out that your board supports dual channel memory,then it's better to go with that as this will help your pc perform better if you put 2 identical 512MB ram sticks in dual channel rather than 1 single 1GB ram. Vincent.
To find out what kind of system specs you are running you can download a program Called Everest Home Edition. It tells you all your computer info for your hardware etc.. It will tell you the Ram type that is compatible with your motherboard. http://www.majorgeeks.com/download4181.html I hope that helps.