It is certainly possible to undervolt the 5870m, but in my experience it doesn't have much practical effect given the way most cards are used. Because the Clevo cards have clocks set at 700/1000 with 1.15v and PowerPlay disabled, they tend to idle in the low to mid 50C's. To combat that, most knowledgeable users downclock to 200/300 or 250/300 at or immediately after startup. In my case, for example, 200/300 clocks result in idle temps of about 42 (core), 44 (memory) and 46/47 (shaders) at 1.15v. Reducing voltage to 1.10v results in 41-43-45/46. Going on down to 1.05v gives me 40-42-44/45. So the temperature differences resulting from undervolting are very small compared to the much larger reductions achieved through downclocking. On the other hand, overclocking capability is significantly impaired. At 1.05v, my Clevo 5870m crashes at just 825/1075. Others may do better, but the Clevo cards seem to need higher voltage to thrive. If overclocking is not a consideration, minor improvements in temp levels can be achieved, but ambient temps will have a greater effect than undervolting for this card. Mandrake, using a Clevo 5870m in a Alienware M15x, reported a 4-5C reduction from dropping voltage to 1.10v. I have never seen a similar result and have not heard of similar experiences, but it suggests that anything is possible. To return to your basic inquiry, the cards can be undervolted but the resulting temp reductions are minimal in my experience. Your results may vary.
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Problems? See this thread at archive.org.
