I purchased additional memory for the Mac Pro yesterday from satech.com [pricing page]. As of this writing, SATech (aka RamFinder) charges $1990 for 8GB (4X2GB), or $249/GB. By comparison, Apple charges $2699 to upgrade to 8GB, or $337/GB. Going the SATech route, one keeps whatever DIMMS already came installed—4X512MB = 2GB in the case of my refurbished Mac Pro.
I’ve had multiple good experiences with SATech over the years, and they stand behind their product. I recommend sticking with “factory approved memory”—the highest quality. However, the FB-DIMMS I purchased do not have the same heat sinks as the Apple-installed 512MB modules, so it’s a bit misleading for satech to claim “This is the same memory product you would buy from your system manufacturer (Apple, Compaq, Dell, HP, IBM, Gateway, Sony and others) directly when you buy the system”. I have forwarded this concern to satech, as I believe they should retract this claim.
The Apple heat sinks on the four 512MB modules do appear that they would be more effective. But see the stress-testing discussion below.