Linked below is a database of modified BIOSes for X58 motherboards patched with the latest microcode (0x1F for Westmere, 0x1D for Bloomfield) against all variants of Spectre. Make sure the operating system is also updated to be fully protected from speculative execution side-channel vulnerabilities (CVE-2017-5715, CVE-2017-5753, CVE-2017-5754).
While most of Nehalem CPUs received a new microcode from Intel, there is an early stepping (C0) that hasn't and therefore still vulnerable at the moment. However, these very few CPUs will also benefit from increased security and stability due to more recent microcode. In addition, the ROMs feature TRIM support for SSDs in RAID0 and extended CPU compatibility table for all motherboards.
Unofficial ROMs! use at your own risk.
ASRock
All of ROMs are mirrored in this location just-in-case.