The big advantage of the 740C over the 640C are the 2 extra digital inputs for other components and using the upsampling technology used in the top line model, the 840C, and ties it with the fine Wolfson DACs used in the 640C.
The 840 uses Analog Devices AD1955 DACs which some claim is superior sound.
It's a personal choice. If you can hear an advantage of one of the other, then that's what you should get. But the jump from the 740 to the 840 is a good $300-400, depending where you shop.