Just saying, a decent keyboard and monitor alone are going to be around $150-$200, adding in windows 8 makes it about $300. Giving you $200 to get a decent gpu (around $150), a good cpu (around $200), a case (~$50), ram ($50-$80), a hard drive ($100), motherboard ($50-$100), and finally, a power supply ($50-$100)
tbh, you'd be better off getting a 17' laptop for around $500 than building your own desktop.
I'm not saying you can't get a desktop, I'm just saying that $500 won't let you play borderlands 2 at a very high FPS, nor minecraft on higher than medium settings.