According to crucial.com, http://crucial.com/store/listparts.aspx?mo...imension%204600
, you have four slots which can each hold up to 1gb for a total of 4gb max. If you have 512mb now, I would think you have two slots filled, each with a stick of 256mb (but I'm not 100% sure). On their site you can also run their scanner which will tell you how many sticks you have now. I would recommend getting two 512mb sticks so you would have a total of 1.5gb.
How much ram you need depends on what you do. If you're just browsing the web, 1gb is probably more than enough, but if you like to do a few things at once, say browse the web, listen to music, chat on instant messenger, watch videos, etc. then you may want 1.5gb-2gb.
"I think there is a world market for maybe five computers." - Thomas Watson, Chairman of IBM, 1943