The 'problem' is if you're running the RAM at DDR3-1600 on any AMD Phenom (x4 or x6) without either a FSB -> 240MHz <or> CPU-NB Frequency -> 2400MHz then it'll be unstable. In addition, quite often the 4GB densities require a BIOS update.
Q - What EXACT set of RAM ; please provide a link.
Q - What BIOS version?
Next, the ASUS M4A87TD EVO won't/doesn't support SLI. However, most of the new AM3+ 990FX do support SLI; you just need to double-check.
The DDR3-1600 RAM can run, but as I mentioned you'll need to OC the FSB or CPU-NB Frequency or in this case the ASUS has a DOCP setting
AI Overclock Tuner -> DOCP
DRAM OC Profile -> DDR3-1600
DRAM Timing Configuration /Enter or Manual
DRAM CAS -> 9
DRAM RAS to CAS -> 9
DRAM RAS to PRE -> 9
DRAM RAS ACT -> 27
DRAM Voltage -> 1.64v~1.65v