Also be forwarned, some 'XT's were actually overclocked PRO cores.
I believe that Gigabyte (surprisingly) GeXcube and Powercolor (not surprisingly) were the main culprits IIRC, but maybe even Sapphire did so. It was mainly appearing in the initial release of the XTs, but there could still be some lurking out there.
However being able to OC above 550 without artifacts is pretty good. I know of many people who've OC'd higher, but nothing's guaranteed with OC'ing. Some Pros can OC above 550 (mine goes to 535) and some true XTs can't, it's always a crapshoot.
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