Well what are you using now? If you don't know, press your windows key and type in dxdiag. Copy and paste what you have here. Running diablo 3 or WoW on medium won't cost much unless you have an 7 year old desktop/laptop. For $500 you can probably max out D3 and WoW.
edit: I'm assuming you're going to use your old monitor/hard drive and dvd drive so I excluded it. let us know if you aren't.
Mother Board and Processor (if you buy this now you'll probably get the free RAM)
A little under $500 total and would play more then just D3/WoW on high settings