...but doesn't this MDD have 10/100 ethernet. In order to take advantage of gigabit, wouldn't I have to put a gigabit pci card in the MDD?...
I thought all MDD's have gigabit, the older Sawtooth models had only 10/100. So if you have DP 1GHz CPUs, you may possibly have a "Quicksilver" rather than an "MDD" but that model also should have gigabit.
This gigabit Ethernet PCI card actually can work in combination with the built-in gigabit Ethernet to get even faster server-type performance via two Ethernet cables. Sounds interesting, I haven't any experience with it.
With all the hard drive bays filled watch out for CPU overheating and auto-shutdowns.