We faced the same question in our working group. I suppose there are few multi-processor workstations left because most people's workloads are I/O bound nowadays, and a six-core CPU is enough for them.
With the current 6-core CPUs, there are only some two-socket configurations (yielding 12 cores) left on the market.
One obvious choice would be Apple's Mac Pro, which really is a very nice machine (cableless HD trays, nice cooling concept, quiet and well-made), but pricey.
Small system builders tend to offer such machines more often, see for example this 12 core box from Thomas Krenn.
However, one thing I note is that single thread performance of most processor series has stagnated over the past two years. So if your jobs are not well parallelizable, I would rather think about one of Intels Core i7 top models -- I don't know about its SMP capabilities, though.