If you are building a rig that would use SLI from the start, you probably have the budget to get an X58 board that supports x16/x16 SLI. If you just want to have SLI as a possible upgrade path for the future, X8/x8 is probably more than enough. I haven't seen recent benchmarks, but the performance difference between x8 and x16 for SLI used to be 5% at most (which doesn't warrant spending 100$+ on higher-end platform IMO).
There are plenty of boards supporting x8/x8, check for P55 boards. P67 is for the upcoming Sandy Bridge CPU which uses a different socket (LGA1155), most website probably don't even sell those yet.