Well first off you'll only be running the two slots in x8 mode with a video card in both because of the maximum amount of available PCI-E lanes on the LGA1155 CPU's, but there's less than a 2% performance hit running x8 compared to x16. (Technically you could spend way more on a NF200 equipped board, but it's not at all worth it unless you're going to be using 3 or 4 cards)
What is your budget for the motherboard? If you can afford it the Asus boards are the best way to go. Something like this: ASUS P8Z68-V PRO/GEN3
But unless you're going to be overclocking past 4.5ghz something like this ASRock: ASRock Z68 Extreme3 Gen3
is a very popular budget friendly board.
I actually have the ASRock in a 2500k system I built a month ago, and it works wonderfully. (2500k @ 4.2, 16GB, GTX560ti) The Asus will give you twice the available SATA3 & USB3 ports as the ASRock (4 each vs 2 each) and be better at high overclocks. But for moderate overclocking the ASRock board will save you a decent amount of $ to put toward other parts of your system.
Either will support two video cards in x8 mode SLI/Xfire.
FYI you're looking something like this ASUS Maximus IV Extreme-Z
If you really wanted true PCI-E x16 to each video card.