you'd need one with 2 displayport outputs and you'd need expensive active adapters or monitors with native displayports. Nvidia has a big benefit at the minute in that you can run 4 monitors natively regardless of outputs. And 4 low end cards is still cheaper than 2 7770s.
I can offer an alternative, however:
buy two of these:
http://www.ebuyer.com/451860-sapphir...i-11190-12-20g
This is a sapphire Flex Radeon 6450 - it uses 2 DVIs and an HDMI to power 3 DVI monitors. Add them to an AMD motherboard with onboard graphics and two PCIe slots (like this one:
http://www.ebuyer.com/280209-asus-f1...ard-f1a55-m-le) and you can use the two graphics outputs from the onboard graphics alongside the three from each Flex card to get 8 outputs. Probably the cheapest way, as you don't need the expensive motherboard with 4 PCIe slots.