Actually, many Z87 boards have the second x16 slot as PCIe2.0x4, which Nvidia refuses to run SLI over.
GA-Z87X-D3H, ASRock Z87 Extreme3 or higher, Asus Z87-A, MSI Z87-G45/G55.
Avoid micro-ATX. You'll suffocate the cards.
The GA-Z87-D3HP is a good example of a board that has two PCIe x16 connectors, but is not SLI compatible.