A lot of the Bigadv heavy hitters around here (Scuba and Arividab) are migrating over to Intel from AMD for better PPD/W.
Magny Cours (AMD 6100 series) have been proven great 4p folding chips. They Overclock quite nicely via the [H] Bios/utilities and are rather power-conscious.
Interlagos (AMD 6200 series) seem to have been more or less skipped by the Folding community. they're power hungry, don't OC well (if at all), and are a terrible value compared to MC chips.
Abhu Dhabi (AMD 6300 series) are the merceedes of folding platforms. Expensive as all hell, and not fundamentally better than any other car... errr chip. They are marginally better clock for clock than the IL chips, a tad better on power, but double if not triple as expensive.
I own a 6386 SE (top tier AD chip) 4p and do love it, but the value just isn't
there. For the money I spent I could have easily built up an E5-4650 Xeon rig much like Scuba's while still having money to spare (maybe a TC rig....). If I were in the market for a 4p right now I'd go one of two ways :
1) Value : Look around and pick up a 6180 system. They're chips all over the place for a fantastic price. They'll overclock nicely on Supermicro boards and net you a solid 500-650k ppd. I'm pretty sure several members here have sets of chips they are trying to sell.
2) High-Output : Do an Intel Xeon rig. The E5-4640 / E5-4650 rigs put out around the same PPD as my 6386SE rig (725-825k PPD) BUT use a lot less power, and are a smidge less-expensive overall.
Bonus way : Build a few 2p Xeon systems. If i remember right Zdngfld's and anubis' systems get ~300-400k PPD and sip power (~250W) A few of those will have a fantastic output, and great efficiency.
Don't let the myriad of information discourage you from building a folding power-house. There are so many routes an individual may take to build a rockin' PPD producer!