1.8V is JEDEC std isnt it? :\
Manuf usually say 2.1-2.3....
End users who has actually tried and not toasted says.... If low ripple, stable volts, a decent ventilation (strap a 80 or 92mm fan on 'em) 3.0v for 24/7 use shouldnt be a problem... However, probaly wont benefit... I wouldnt mind using 2.8v for daily... (think it was 2.7v I used for 24/7 back when I used DDR2)
Nor would I mind using using 2.35v for daily on DDR3... and if it'll benefit daily, I'd might even go 2.4-2.5v daily...
But hey, I'm insane ;)

