Let me say that I have built many dual cpu socket 'Hack' Pro's and, for the money, Supermicro has never steered me in the wrong direction. I am typing this on one of my main HackPros which has two 5645 hex-core Xeons running on a Supermicro X8DAL mobo and I couldn't be happier. The performance for the price range (or bang for your buck) is unparalleled when it comes to these boards and somehow they seem to just agree with the Mac OS - at least with the socket 771 and 1366 mobos, i can't speak beyond that.
On the topic of overclocking - try to avoid it with any dual Xeon rig for the very reason that the chips and mobos in this category are designed to run as stably and with as much power as possible - or in other words, they are not designed to be used in the everyday gaming rig. There are TONS of new boards with the LGA2011 sockets that allow the fastest i7 CPU's to crank to well beyond their means with as many as 8 cores per die that do not have to communicate with another CPU to load-balance. Case in point - if overclocking is the straw that breaks the camels back in this scenario, I would not use the Xeons even with an SR-2 board - there is a reason that these types of mobo's are few and far between.
If you want my advice (if it matters at all), here are the parts that have served me well for many years in the dual Xeon HackPro arena:
NOCTUA!!! CPU coolers - (these sinks/fans will destroy any other fan-based cooler; more CFM per dB than any other cooler outside of a custom liquid build)
Kingston ECC Buffered DDR3 RAM
and a very powerful PSU...1kW should more than suffice.
These are just my two cents - the ramblings of someone who has been down this road many times since the early days of Tiger by upchuck. I hope it helps.