Those lines are called artifacts and it means you are pushing the card to far. You should back off untill they dissapear. They arnt healthy.
Are you just using the stock cooler? Does it even cover the memory modules?
Artifacts at a low overclock are usualy caused by heat. I would advise purchasing an after market cooler like the zalman VF700Cu for around £25 if u got the cash.
That should make it a darn site better. For a cheap alternative if the memory isnt covered use thermal compound to stick little pieces of sheet metal onto the memory to act as a heat spreader and also atatch a 80mm fan onto the card to give it a lot lot more air flow. I did this with a really really shit radeon 7000 and made the speed increase from 140/140 (crap) to 230/210 (still shit lol but a lot better). Used custom heat spreaders and an AMD 64 stock cpu cooler fan.
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