What is the most powerful graphics card that fits in an Alienware x51 r2 case?

I want to know what to upgrade to as most say only the gtx 970 is possible to fit and is the most powerful but can you fit a more powerful card in thi

I want to know what to upgrade to as most say only the gtx 970 is possible to fit and is the most powerful but can you fit a more powerful card in this case?

or:I want to know what to upgrade to as most say only the gtx 970 is possible to fit and is the most powerful but can you fit a more powerful card in this case?

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