With a config as follows:
Amd R9 280x x2 700$
Amd FX8350 200$
midrange mobo 100$
16gb DDR3 ram 80$
Corsair 850w Gold certified psu 120$
Full Tower case 80$
-max everything possible at full hd probably for the next 4 years
-mine effectively at 1200-1400 kH/s no problem
By doing so in your spare time, you'll recoup the cost of the system (1300$) in 3-4 months tops (if you're unlucky) or a couple months if lucky.
After that... it's all nice tidy profit.
Obviously I'm not suggesting Bitcoins at this point, but there's tons of lesser known and new Crypto currencies launching every other month.
Lots of people got fat doing this, early adopters of Bitcoin now are all millionaires.
A bitcoin was valued about 1$ a year ago, now it's worth 780$.
Litecoin just a few cents, now it's at 20$.
Happy 2014 pc gamers.