Based on what you have said, I think your best value is to get a Core i3 with an Intel H67 Media Series motherboard (like the DH67BLB3 board that Andrew reviewed). The reason for this recommendation is that the i3 graphics will fulfill your requirements and an Intel Media Series board will have an Intel NIC.
If you are happy with your C2D system, there is no reason why you can’t keep it.
