I currently have an MSI z77a-g41 board. It supports crossfire, but not SLI. (some places claim it does support SLI) The second PCI slot only runs at x4 so even if SLI is supported, it won't be worth it.
I know that 2x gtx560 will not be quite as powerful as a single 7870 but like I said I do not play alot of hardcore game and I like to tinker.
I will be buying another gtx560. (cheapest one I can find on ebay that is still 1gb, 256-bit. Don't care about manufacturer.) I have seen them go for as little as $80.
I just need to decide on the cheapest and best quality motherboard for SLI. I use an i3 3225 currently. I plan to get an i5 but do not care to overclock. (GPUs yes, CPU no) So, an h77 board would be fine as well as a z77 board.