1) Since you're running it at 1066 and thus JEDEC speeds, any DDR3 RAM should be able to run at the same voltage and latency settings that you're running your RAM. Not matter whether it's 1.65V overclocking RAM or low voltage RAM.
2) The only problem might be that the two kits might not like each other. When you're overclocking it's sometimes a problem to fill all memory banks, but since you're not, only memory compatability should be a problem.
I actually have the same kit of memory, but my EX58-UD3R isn't running at the moment or I could test some RAM for you. Unless someone actually tested it it's very hard to say which current memory runs fine with yours. I would probably get a 3x4GB from a reputable vendor (Corsair, Kingston, Crucial etc.) to make sure the SPD is properly programmed and just test it. You also might be able to sell your RAM for a nice amount, 1600 CL7 is actually pretty nice, you might not have to pay much on top of that to get a second 3x4GB kit.