Hmmm... I don't play World of Warcraft myself... but as far as I know, the game is not that graphics-intensive... So my advice would be to upgrade to a faster CPU (and perhaps motherboard, depending on what CPU you pick).
That game is not very demanding, the only lag I got was is open beta from server lag with too many people on. 512 RAM should run it fine I was running on a P4 1.6ghz 768mb RAM and a geforce 2 32mb video card and could still alt+tab and surf the web.
That game is not very demanding, the only lag I got was is open beta from server lag with too many people on. 512 RAM should run it fine I was running on a P4 1.6ghz 768mb RAM and a geforce 2 32mb video card and could still alt+tab and surf the web.
Edit: If you get a new processor you will probably need a new motherboard and be able to use new RAM.