A manual mild overclock will work in games. With anything higher than mild overclocks, Furmark will work but games will mess up. ATI CCC puts the overclock far higher than it will stably work.
One interesting point: After weeks of trying different overclocks, I've never seen any artifacting. The will work perfectly fine for a while and then the screen will freeze and go blank. There seems to be a fine line between working perfectly and having to manually restart the computer. Does this seem odd to you?