Sounds to me like you've already got past the hard part, by getting it to boot with 2 different brands of memory. If you run a program like CPU-Z , it will show your timings. Run a stability test (I like Everest Ultimate Addition 5.50 http://www.lavalys.com/support/downloads it's a trial addition, but it's free to download) if it passes you should be good-to-go, unless you plan on OC'ing.
PS: your memory can only run as fast as the slowest stick (which in your case you will never be able to tell)
To hard-set the voltages go into the Advanced>(mine says) JumperFree Configuration>AI Overclocking (Auto) set it to Manual. Scroll down to DDR Voltage and type in 1.8 (Warning: don't forget the point!! in 1.8) hit the F10 key>Save changes (yes). Your memory voltage should now be set to 1.8v.