try to find this:
dual LGA1366 motherboard with 2 pci express 2.0 x16 slots and at least 12 DDR3 dimm slots
2 Intel xeon E55x0 series cpu's (at least 2.4 GHz)
2 sets of 12GB (3x4GB) memory
this would give you 8 cpu cores with 16 threads, a motherboard that supports crossfire (no sli), and 24GB of memory (just make sure you split it between the processors)