FBDIMMs are a power hog. Each stick uses like 8-10w of power.
Pretty much everything that is pre-1366 for CPUs is going to be a power hog as well. Before the 1366 socket CPUs were designed around maximum performance and not good idle power usage.
So whatever savings you think you are going to reap by buying used you'll spend on cooling that beast. People selling that old stuff know about its limitations. They mark it down just enough to give the uninformed the impression it's a great deal. But long term its not.
Also keep in mind that motherboard is an Extended ATX. It's not going to fit in all cases, so you may be forced to buy a case just to fit that thing.
I'm not sure what hardware you are looking at, but you can get a Supermicro X10 + G3220 for about $225. That CPU is plenty of power for your system unless you plan to do encryption, transcoding, or setting up compression for your pools.