memory up grade help

I have 512 mb on my pc now, but have two slots ( only one being used). If I add another 512 to it I should get 1 gig right? now do I need too replace the old one or can I get one memory card and just add it to the empty slot. Trying to play WoW and don't have enough ram.

PC INFO.

HP a720n

AMD athlon 3200+

200 gig HD

512 ram

video card: Via/S3G UniChrome IGP

I know the pc is a little dated and needs a major upgrade but, the memory is probley the first thing I should do.

Put another stick in the empty slot. A 1gb stick of ram will get you to 1.5gb, which is plenty for that game.hartsickdiscipl