As far as I know the Athlon 64 3000+ in socket 754 is a 2.0 GHz CPU, and the Athlon 64 3000 + in socket 939 is a 1.8 GHz CPU, so it seems logical that the older box compensates other deficiencies by a 10% slightly faster clock.

Personally I have never found that the hard disk speed was a bottleneck, unless of course there is not enough ram and swapping occurs, or the hard disks are incredibly slow. But on a renderfarm anything can happen :)