I think some memory modules come with thermal sensors. However, it isn't true for most of them. The absolute red zone for DDR2 is somewhere around 2.2V. I wouldn't be too sure about that, though. Each manufacturer quotes differently. What I am sure about is that 1.9V is relatively safe. DDR2, by default, runs at 1.8v anyway. Although 0.1V is a "huge" change in terms of RAM voltage, it's still well within tolerable bounds.
It's very slightly risky, but you can try it with the 1.8V modules only. Then you can decide if you want to bet and put in the 1.9v modules along. Otherwise, you can try to sell it off and buy another pair of the same 1.8v modules.