AM3+ 1600 to 1866 memory OC

My specs are as follows:

Greetings,

Fx-6300 @ 4.3 w/ 200 x 21.5 (stable under P95 and has been running for months)
GA970A-UD3P
4x4 gbs of G.Skill Ripjaws X series 1866-9-10-9-28 http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Sapphire R9 290 Ref.
Antec HCG-850m

1.) What is a safe limit on the DRAM voltage with that mobo and ram? Auto is at 1.5 and I pushed it to 1.57v.

2.) What is a good stability test besides memtest86? I have been using Prime95 custom and setting ram use to all but 1gb for system use but one core has been failing, even when using just 2x4gbs. I don't if that's the best way to test it.

Don't mess with memory, the ram auto runs on 1600, just want to setting 4x4 gb at 1866.

Thanks,
Ben
 
It didn't boot with XMP, so checked with Gigabyte and found that the mobo won't run at speeds higher than 1600 with more than 2 dimms.

However, with 4 dimms, I am running at 1600 11-11-11-28, and it doesn't seem stable. Any ideas to best tweak it to stabilize at 1600?
 
It seems that increasing "NB core" voltage destabilizes the CPU overclock. At 1.25v Prime95 SmallFFTs failed in 7mins, @1.2v failed in 30mins, 1.186v in 1hr 40mins, and 1.181v was stable after 4+ hours.

I don't think I have enough head room to increase to 1.3v.