The C7-D is about the same speed as a PIII-800. However, it will use less power than the Athlon.
The C7/MB combinations have ridiculously low power consumption profiles that make the Atom combos seem hungry. For any media or heavy general purpose context switching stuff, the C7 is indeed around the same as a P3/P4 at ~70% comparable clockspeed. However, because of its built-in encryption hardware, it outperforms even non-hardware-encryption i7 CPUs at ~2x the clockspeed at a fraction of the watts. So for any sort of NAS-wth-disk-encryption or router-with-SSH/SSL-encryption, it's a perfect CPU choice.