Alright, if my guess is right, this should be fine, 2 of these would suffice
Newegg.com - CORSAIR ValueSelect 512MB 184-Pin DDR SDRAM DDR 400 (PC 3200) Desktop Memory - Retail
Although wait for a second oppinion... idk if its an old enough proc to be on a mobo that would take 168 pin modules.
Also, are you using a graphics card? or onboard?
WoW doesnt take much, but your comp seems dated, when did you get it?