you can't expecct incredible gains in 3DMARK06 scores by just overclocking your GPU... little increases means little increases in scores.... 50 points is quite a lot to gain
anyways, are you sure ATI Tool isn't working? do you have the latest version or is your card just not supported?
also, its not always safe to just run a program and hope it doesn't crash your pc... you need to be monitoring temperatures and checking for actual stability
artifacts are little problems on your screen such as small white pixels that show up when your GPU is having problems, when you overclock too much, the picture that shows up on your screen may tear, pixelate, get artifacts etc. that is an indication your card is not getting enough voltage or is getting too hot