You did not say how many WoW you are running on each machine, but RAM cures all ills. If you can put in 8 gigs, do it. You already have the 64 bit OS.
Onboard graphics is not a good setup, almost any Video card would improve your system. Buy what you can afford but remember you can always move a good card to your next system. Low end, look at a G260 or a 48xx card, for more than WoW needs but other games can use move up to the 57xx level or higher.