The HD 7850 IceQ X features clock speeds of 860 MHz core and 1200 MHz (4.80 GHz effective) memory, in line with AMD reference clock speeds, while the HD 7870 IceQ X features 1000 MHz core and 1200 MHz (4.80 GHz effective) memory, also in line with AMD reference. Then there's the HIS HD 7870 IceQ iTurbo, featuring a gargantuan new cooling solution never before seen. This cooler uses an aluminum channel heatsink, to which heat is fed by four 8 mm-thick copper heat pipes, through which air is blown by a lateral-flow fan. Interestingly, this card, too, sticks to AMD reference clock speeds.
source: http://www.techpowerup.com/161730/HIS-Announces-its-Radeon-HD-7800-Series-Graphics-Cards-Including-Three-IceQ-X-Models.html