I'm using an ASRock H77 Pro4-M right now, you can use 2 Cards without any extenders, haven't tested if you are able to extend the pciE 1x yet but you can still use 3 Cards with 1 or 2 16x extender. You can either just extend the middle slot but it will be hard to place that card xD or extend the two bottom slots and move the cards next to your mainboard.
Next incoming board is Gigabyte GA-H61MA-D3V, you can use 1 Card out of the box and the other slots are working with 1x to 16x extender (found the info in a bitcoin forum, can verify tomorrow) it will allow you to run 4 Cards on a single board.
Go for the cheapest 1155 CPU, thats usually a Celeron 46x, take 2-4 GB RAM and thats it.
Upside with the 7790 is that you can use a way cheaper PSU since the 7790 only uses one 6 pin power connector, every PSU above 400W should have at least one, one card uses around 80W so you could run all 4 on a decent 500W PSU.
I should start selling rigs...