MaDef is right, I would get 2-1GB Memory modules to replace the 512MB you have, with todays pricing. I would also use PC2-5300, as it is a little faster.
Newegg is a very reputable company. I see they have
2-1GB sticks of gSkill (which is quality memory) for
$24.99 w/free shipping.
If you go to:
http://www.crucial.com
Plug in your specs, it tells you the type of memory it takes.