I am still pretty clueless about F@H but I have been pondering about building a good PPD/Watt machine myself.
My current setup is:
Turion MT-37 (S754, 2.0ghz) on a MSI K8MM3-V
Radeon HD 3850 AGP
Seasonic S12 330W
2 GB RAM
~80W idle (cool'n'quiet enabled, 1.05V CPU, 0% CPU and GPU) and ~160W load (1.35V to 1.40V CPU, 100% CPU and 99% GPU). There isn't much tweaking I can do via BIOS but if I limit GPU client to use less 100% of CPU then GPU usage drops to 60% or idles often with occasional peak to 99% as opposed to almost constantly at 99% (according to Catalyst Overdrive panel).
For the past week or so, stats show I am getting around 1400 to 2200 PPD (~1500 PPD avg?).