I have found that W7 Aero desktop does suck around 100khs on a 2560x1600 resolution with a dual link DVI. This system has 2x7970s and the GPU having monitor connected to is doing only ~600khs while the other GPU does +700khs.

My completely headless miner does 740khs on all cards. Gigabyte Windforce 3 OC 7970 (GV-R797OC-3GD) rev 2.1 volt locked cards here @ 1060/1460.

cgminer 2.11.4 --scrypt --lookup-gap 2 -I 13 -w 256 -g 2 --thread-concurrency 8192
Catalyst 13.1