For starters, you will need a P55 based motherboard that specifically says it supportsl Nvidia SLI. Just having 2 PCIe slots will not work. Also just because it says it supports ATI Crossfire, does not mean it supports SLI. The board should also come with it's own SLI bridge accessory to connect the two cards. Don't worry about splitting the x16 lane into x8 and x8. Hardocp.com has run tests to show that is not an issue.
Most of the newer motherboards have USB 3.0 and SATA III, both of which are nice, but by no means necessary.