9800gt already overclocked, is it ok to overclock it?
gigabyte 9800gt GDDR3 512mb OC edition
is it safe to overclock it even it's already factory overclocked?
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Yes, you may overclock it even further but there will be much less headroom since it has already been overclocked. In my opinion though, the core clock is already quite high so I'd just increase the shader/mem.