ASICs, own risk, stupid to mine on GPUs, yadda yadda and so on. Anyway...
What you're looking for is the cheapest mobo that can take two of the cards. I think they use pcie, so go with that. I'm fairly certain that cards designed to run on pcie 2 can run on 1 as well, and the decreased bandwidth doesn't matter too much to you. once you've picked out your mobo, grab a gig or two that fits into it, and the cheapest processor that fits into the socket on the mobo. That's... basically it. You'll want a power supply that can supply enough power for everything (there's a power supply calculator online), and a tower to house this all in. Try
http://pcpartpicker.com/ to see what can be bought new, and then look for old computers with compatible ports that people are getting rid of or throwing away. Reusing old hardware will make your power usage higher, but initial investment cheaper.