There are no PCIE 2.1 cards from nvidia yet, Even the brand spankin' new gtx570 is only PCIE 2.0, there are known compatibility issues when using a PCIE 2.1 card on a first gen PCIE mobo, 2.1 is just a stepping before PCIE 3.0
Sorry but you'll have to return it, and get a 2.0 card.