What voltage for RAM?

Hi,



I have an old system, specs-



CPU - AMD Phenom II X4 955 3.2GHz

Motherboard - Asus M4N78 Pro

RAM - Corsair XMS2 4GB (2 x 2GB) DDR2 800MHz (Link - http://ift.tt/1DheBYk)

GPU - XFX GTX 260 Core 216

HDD - Seagate 500GB 7200RPM

PSU - Seasonic X-650 Gold



I bought another 4GB of RAM from G.Skill, Link - http://ift.tt/1DheBYm



Now, here's the problem. I didn't realize mixing RAM would cause so much hassle. Corsair has a rated voltage of 1.8V and the G.Skill is rated for 1.9V.



G. Skill doesn't work under 1.9V, therefore I have kept them both at 1.9V.



I would like to know-



1. If overvolting is fine for the RAM, although not much I just want to confirm.



2. I have tried the pairs in all mixed slot configurations and compatibility issues still exist - like for instance sometimes it won't even post. When it does, it works fine, even under gaming.



Currently, I have manually set the timings and voltage in BIOS. I would like know if there's anything more I need to know while mixing RAM.



Cheers!

What voltage for RAM?