Or otherwise look for the processing power which meets your requirements - regardless of cores or number of physical CPUs. Then you can look for a board which supports 8 x 4096 DDR3 1333 Mhz RAM (or more) and appropriate processor(s).

Between 8 and 12 slots for RAM is all you will probably find, but not necessarily for DDR3.

A two processor board with more than 8 DDR3 slots (12 or 18, probably) is sounding an awful lot like a server board, but Xeon processors may not be optimal for whatever it is you do on your PC which requires such amounts of RAM and processor power.

You can try tailoring searches, like:
http://www.google.com/search?ie=UTF-8&oe=utf-8&q=DDR+3+8+slot+board
But check against to total RAM - you may have 8 (or 12) slots, but only support for 1 or 2 GB per slot.

It is difficult, but you have difficult requirements. You may want to look at http://www.tomshardware.com/reviews/Components,1/Motherboards,2/ to help sort out what meets your needs best. Once you have a better idea, you can ask questions in the forum there, where a community dedicated to building PCs can better and more readily answer your questions.

Still, check back here. Some members are quite knowledgeable in the PC-building department. ( Col, I'm looking at you, here. )