Download and run PC Wizard (free).
It will tell you how your slots are populated without opening your computer and if there’s an SPD chip on them you will get mfr info too.
If you have one slot presently populated with a 1 gig stick, then try to match that for dual channel enablement, as previously mentioned.
If you have 2 x 512MB then you will have to start over if you want 2 gig.
Go to a major memory supplier like Crucial, Kingston, Corsair and use their [B]memory configurator[/B]. Enter the appropriate info in the fields and the correct memory module part numbers (for your comp) will be shown.
Once you have the correct part numbers in hand you can go shop at a reseller of your choosing.
With a matched 2 x 1GB module set, make sure that [B]dual channel[/B] is enabled in your BIOS, if that option is present.