i dont see any prob with the bottlenecking.. although it does bottlenecking, the main cause would be ur processor not mobo.. as u said the mobo only support ddr3, it actually means ur physical desktop ram, not ur gc vram.. the gtx760 gddr5 still can be supported and perform well..

let say if ur processor really bottleneck ur gc, i think only 1-5fps drop la.. wont be a major issue for u to concern. Unless u really wan to unleash the full potential of gtx 760, then u gonna upgrade both ur proc and mobo. Proc=4 cores above, mobo= at least can support pci-e 3.0