It's not about load. The problem is drivers and 6xxx cards. I have two 6970 on two PCs that make PC freeze when miner is running and any video is started. And I have 3 5870 cards on another two PCs which run perfectly fine even with hardware acceleration enabled. I'm absolutely sure it is related to compatibility between 6xxx cards and drivers. Whatever load there is, it should not cause computer to hang.
I have twin 6970s on a win7 x64 machine. each card is doing about 406mhash with "-f 60". I uncheck hardware accelleration and I can play embedded videos just fine. EDIT: with catalyst 11.6 drivers.