Honestly? I'd just spend the extra $40 to save yourself a potentially huge headache. Running two different kinds of RAM with voltages that far apart has trouble written all over it.
With all four slots filled, each stick often requires more voltage to run properly, so you may have to bump it up by .1 or so over manufacturer specs ... I don't know what motherboard you have, but most I've ever heard of only let you choose a single RAM voltage for the entire system. So you're looking at running close to 2V and two of your memory sticks that are rated for 1.65V. Not 100% sure how much excess voltage it takes to damage that RAM, but I wouldn't recommend it.
Also, unless you really like playing around with settings, most likely the second set is just going to drag down the performance of the one you already have. Why bother with all that for 40 bucks?