I was at the AMD PII demo is boston, and for air cooling, 1.55v is safe (with obviously good aircooling). One of the demo machines with the STOCK amd heatsink could take 1.55v. The side panel of the case was opened though. Temps were still very acceptable too (under 50°C). This machine was OC'ing to around 3.8Ghz stable. Mind you this was with the stock heatsink. Strap a TRUE and a good fan on there and...
The phase change machine was easily stable at 4.8Ghz, with 1.45v! Temps about -35 IIRC.
For LN2 macci was using 1.9-1.95v for 6Ghz numbers.
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