they are just different manufacturers with a different spin on the reference design.
best makes of graphics cards imo are xfx,asus,gigabyte some do overclocked versions, some do superclocked versions. I tend to buy factory overclocked cars and never bother myself.
But with the newer graphics cards and Bioses these days overclocking is easy. I have heard of people overclocking the gtx560 to 950MHz without any bother but as with any overclocking and changing of stock speeds it does have risks.
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