actually equal or better performance if possible but the reseller has the final decision. If they cannot offer a suitable replacement without undue cost to themselves they can insist on issuing a refund.Its meant to be equivalent or nearest greater value.
But for god sake man just take the money. you could get near enough the same performance from two of these (http://www.overclockers.co.uk/showproduct.php?prodid=GX-103-MS) & pocket the extra £200