Need help to build a mining rig (Noob)

So, I know a few things about Bitcoins, Litecoins and Ether.
I earned some bitcents by solving captchas and so on, but I don't really like it ^^

That's the reason why I'm here: I want to mine on my own.
So, I looked up some GPU's and on cryptocompare I found that one: AMD Radeon HD 7950
I would buy two of them.

Would you take this card or another, better, with a longer payback period?

And... I really don't know which mainboard / RAM / CPU I shall take.

Mining OS? I heard about Ubuntu (which I'm currently using here) and Windows 7
What's the better one? I have some skills with Linux

I'm not going to mine Ether for profit, but I don't want to make loss.

Greetings from a No0b