What type of DDR3?

[dawgtuff]

I'm doing a new build. I've decided to go with a Phenom x4 955 CPU, a ASUS M4A87TD/USB3 870 mobo, and 4gb of G.Skill Ripjaw DDR3. I want to lower the timings of the ram as much as I can. I know the Ripjaw is at 1.5v. So, should I go with the 1600 or 2000 of the Ripjaw? Or, is there a DDR3 out there that runs at a lower voltage so I can underclock to get better timings?