Man, the more I dug into this the more I realized how much I have missed! Looks like the real sweet spot (which you nailed) are 4p AMD Opteron rigs. I am looking at making one myself to put in my closet as a dedicated folder and then only turning on my i7-920 desktop for what I originally built it for - gaming.
Here are some resources I found:
PPD/watt measurements from the [H]orde: http://hardforum.com/showthread.php?t=1533433
Database of Folding Machines and their performance: https://docs.google.com/spreadsheet/ccc ... DcHc#gid=0
Guides for 2P/4P/GPU folders to optimize your kit: http://hardforum.com/forumdisplay.php?f=133
Lastly, a link to Sales/Trades thread that may prove useful for getting online on a budget: http://hardforum.com/forumdisplay.php?f=111
I think hunting the PPD/Watt is going to be priority one, then following up with cost to get online.
Research on Opteron Mangy Cours Procs (61xx series...62xx series historically are avoided due to higher power draw and lack of overclock-ability)
6128 - $100 - 2.0 stock GHz - 8 core - 80W - 257 oc bclk - 187K PPD Stock - 245k
6168 - $299 - 1.9 stick GHz - 12 core - 80W - 230 oc blck - - 420k PPD Stock
6172 - $250 - 2.1 stock GHz - 12 core - 80W - 240 oc blck - 55deg - 475k PPD Stock - - 595WNote: Found a 8-pack of these on "the bay" for $218 a chip...if more than one person does a 4p build at the same time, purchasing power could save us all $$$
6174 - $315 - 2.2 stock GHz - 12 core - 80W - 230 oc bclk - 60deg - 510k PPD Stock - 580k
6176SE - $289 - 2.3 stock GHz - 12 core - 105W - 231 oc bclk - 50deg - 550k PPD Stock - 680k - 665W
...just a little "light reading" in some other forums to get some best-case scenarios.