Mixing two very similar RAM kits- "will it blend?"

Yes, however you will either have to set the voltage to 1.8 or 1.9. I think the 1.8V modules can tolerate just 0.1V more. Just make sure you have good cooling.

So will putting it at 1.8 just make my 1.9v RAM slightly less powerful? That seems like the safer route, I'd rather have 7._ gigs of ram than 8 gigs of RAM that are gonna burn out.

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.

Any good way of seeing my memory temperatures while it's running? Nothing in my PC usually hits over 40c, so I'm not overly worried as it seems my case fan keeps things nice and cool.

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.

Best answer selected by r_manic.