I currently have two 2gb sticks of Corsair XMS2 DDR2 6400C5 ram in my computer. Timing is 5-5-5-18, 800MHz, voltage is 1.90
I want to add 2 more 2 gig sticks of ALMOST the exact same ram- It's Corsair XMS2 DDR2 800MHz, 5-5-5-18, but the voltage is 1.80 (it's just a newer version of the same RAM, I guess the voltage changed...)
No, it doesn't work that way. It doesn't make your RAM "less powerful". Rather, it might not be able to run the RAM at the lower voltage. Memory capacity is constant; the voltage does not affect it. I think it's better to run everything at 1.9 to be sure that everything will work. Again, just make sure you have good cooling.
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.