Other than being registered ECC, is there anything special about it?
I was given 4x1gb of DDR2 a little while back that I've never used, but was told it was brand new and was ordered for a server that ended up dieing before it was installed. I also picked up a half dozen or so rack mount servers a while back...I'm using two of them and have plans for a third, but am keeping the other three for parts. I think these are all DDR-2, and of course are registered ECC.
As per OWC, the 2006s have 8 slots, so with 8 1gb sticks(which I'm sure I could come up with) I'd have a decent amount of ram.
Like I said, though, I'd rather spend the money for a 2008, since it does officially support Yosemite(or Mavericks, which I prefer for the time being).
Everymac.com states they originally shipped with "667 MHz DDR2 ECC "fully-buffered" FB-DIMM memory (with a heatsink design that is a bit different from generic FB-DIMMs." Other than the heatsink, and the higher prices, I don't know if there's anything else different about them.
I was looking at one of these older Mac Pros a short time ago on my local Craigslist. A company had a number of them for sale ranging from $300 to $450US. The only difference between them was the amount of ram--2GB to 16GB.
I did a little research, and found an article where the author mentioned the ram for these particular models was more expensive, and should be considered when purchasing one. OWC's prices seemed to verify this statement. In the end, I dropped the whole idea and have kept using my MDD's and Imac G5's.
$199 seems like a reasonable price compared to prices I see them go for on eBay and CL. If your ram does work, then you've got plenty.
I agree with you, though, that a 2008 model would be better because of the option to upgrade to a newer OS version. Of course, they cost more.