With an X660, I'm not convinced that power is not an issue.
GTX 660 Ti Roundup (ASUS, EVGA, Gigabyte, Galaxy, MSI)
and
AnandTech - The GeForce GTX 660 Ti Review, Feat. EVGA, Zotac, and Gigabyte
Both show ~300w for a single card. I can imagine that running SLI will hit the 600w mark, add the CPU and you will push your Seasonic past its limit.