That motherboard can support up to 4 cores. The best chip for it is the i7 3770, which is still pretty average for Archicad, but very cheap on ebay.
I play with old systems for my students. The very best is the 1366 motherboards. You can buy older 6 core Xeons that go super fast. Best deal right now is the x5675 for about $100 on ebay. With minor overclocking, they'll go as fast as the new expensive systems.
You can even get a dual cpu motherboard and 2 x 6 core chips for a super fast machine.
Pair that with a faster 4gb gaming card and you should be fine