What is your budget???
and mainly psu's with 4 pcie connectors Mainly 850w to 1200w psu.
you can get two cards working by connecting first case to 12v rails and for second card you can use MOLEX TO PCIE ADAPTERS. these adapters are on newegg for 5$.But it depends on you psu and graphics card that how many power they need. if you give the name of cards you want to use in sli or crossfirex than i can suggest a psu for you.