I recently acquired an original Mac Pro tower dating to 2006 as a gift. Supposedly OS X Lion was the end of the line for this model. However I found some online sources stating that with a bit of hacking, this Mac Pro could run the latest OS X version, El Capitan. Check macrumors.com if you're interested in finding out how to do this. There were some prerequisites however: Increasing RAM from the 1GB minimum that originally came with this computer. And upgrading the graphics card from 256MB to at least 512MB.
I initially thought 8GB of RAM should be sufficient and so started looking for RAM source. OWC/MacSales had an excellent web site for determining the kind of RAM necessary. Their's was one of the few web sites that explained that this Mac Pro required RAM with massive heat sinks to prevent overheating due to the architecture of the RAM cards. So I ordered the 4x2GB modules for $99 that was quickly shipped. Installed and along with the 3GB that came with the machine gave me a total of 11GB. This worked perfectly with OS X 10.7.5.
Further research revealed that due to some technical aspects of this computer being made to mod the computer to run the latest OS X, 10.10.3, a minimum of 12GB of RAM was needed and that the original 512MB RAM modules would not work. Armed with the knowledge about the necessary heat sinks, I was able to find 2x4GB RAM on eBay for only $32, a much better bargain and increasing memory to a total of 16GB.
As long as I was going this far to upgrade the computer, I bought a 512MB Nvidia graphics card (GT8800) on eBay for $85 and a 120GB SSD to use as a boot drive. The computer already had a 320GB SATA boot drive, a 500GB scratch drive and 500GB RAID. So I replaced the 320GB rotational boot drive with the 120GB SSD from OWC/MacSales. Hacked the boot.efi with information found on the Mac Pro 2006 forum on macrumors.com and installed the latest El Capitan OS on the SSD.
So now I have a blazingly fast Mac Pro running El Capitan that can drive two high resolution monitors and has lots of storage space for video and photo editing. There are a few features of the latest OS that still won't work because of hardware limitation (Handoff and AirDrop for example), but that's not a problem for what I'm using it for.
So although the prices for memory may not be the best you can find, the technical information you can find on OWC/MacSales site is invaluable as well as their being an excellent source for specialized tools for upgrading your hardware.