Even the thread is rather old, I have to clarify the above thesis is NOT correct.
You CAN have a dual 6 core rig. You can use intel xeon 56xx series. Asus, Intel and EVGA has such main boards. Such a rig is relatively cost effective , because the CPUs are rather old and cheap.
The best combination is dual X5650 on EVGA-sr2 running @ 3.80ghz. You can set this very easy and it’s 24/7 solution . This way you have 24 threads running constantly @ 3.80ghz.