Christian Behrens
Supporting Actor
As has been mentioned, it's actually server-class memory, with error correction (ECC) built-in and their own heatsinks.
Incidentally, a friend of mine who got a Mac Pro setup similar to yours a few days ago was lucky enough to order another 4 GB from OWC for about $420. Apparently it was mispriced on their website, but they sold it to him for that price anyway .
Check here for RAM prices:
DealRAM
And this is what my lucky friend got:
He deserves it, though
-Christian