I'm using two EK Nickel/Acetal blocks on my 5850's and temps are great. Idle temps are about 30c on both GPU's, VDDC's are about 34-35c at idle on both GPU's, both clocked at 850/1250 at 1.112v.

10 loops of Crysis Warhead bench Frost at 1920x1200, 4xAA, Enthusiast settings, GPU temps reach 38c, VDDC's reach 48c.