because you can usually get more of an OC with fastwrites off. But it varies with every system. Bench it for yourself though. You will not harm anything by turning it off. So turn it off and run 3Dmark'03 a few times. Then turn on fast writes and do the same. Compare the scores and let us know.
I have fastwrites and sidebanding enabled without any problems. My advice is to disable them till you get your peak OC, then enable them. If they don't cause a problem then leave them enabled.
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