eh, it might so long as it has the same or very similar latencies. what you might wind up having to do, to get it all to work together, is boot into the bios and manually set the latencies and voltage to match the poorest performing stick.
ex 1x4gb ddr3 1600 cl10-2t 1.5 and 1x2gb ddr3 1600 cl9-1t 1.65 in a system together, you might have to manually input a combination of the 2 sticks in the bios. pretty easy really. in this example, i would input the frequency (1600mhz), the voltage (1.65), and the timings (presumably 10-10-10-34-2t) because these values represent the poorest of these sticks combined jedec timings from the manufacturer.
however, in order to do that, you might have to boot with only 1 of the sticks of ram in. in the example machine i was using earlier i would boot up only with the 1.5v stick because generally ram will be better off with more voltage, than less...within reason. i would expect a 1.5v stick to boot with 1.65v, i would be surprised if a 1.65v stick worked with 1.5 unless you really backed off the frequency and timings .
i guess that covers it all. any other questions?