NAS don't need CPU power. I run a file server for 400 people at work on a single core AMD virtual machine and it runs about 10% utilization.
I don't see much point in upgrading. For $100 including MB and RAM (you say you have RAM, but I doubt it's DDR3), you are going to end up with like a bargain basement Celeron or AMD A4 anyway, which isn't a whole lot faster than a C2D.
Waste of money, honestly.