MAX voltage should be 1.65, although lower is better, less temp etc.
In terms of speed, the mnemory controller in an i5 750 is at 1066. If you put 1600mhz ram in it will mst prob nuderclock itself to either 1333 or 1066.
The best thing is to check the compatibiliyt lists for one. Also it really does depend what you are going to use the system for...overclocking?gaming?CAD programs?VMs?
for gaming i would say the CL is more important than overall speeds.
If you plan to overclock then you would prob want a higher speed..
if your doing VMs, or other such stuff again prob speed...
Although teh memory controller is in the CPU, teh motherboard anufacturer shoudl provide a memory compatibility list see here: http://www.gigabyte.com/products/product-page.aspx?pid=3347#ov on the right go to the memory support list