You paid top dollar for a 2010 Mac Pro that does single-bit memory error correction and double-bit memory error detection on every read from memory. The more memory you install in your Mac, the more likely you are to have a memory error occur. Without that logic and the extra bits working, your Mac will continue to use modules that will continue to make errors, and the errors will propagate.
When RAM memory errors occur and are allowed to fester in this way without detection, they are beastly to find and fix. Often the damage propagates through files and throughout your entire system.
If you need a great deal on memory, shop harder for the correct ECC memories appropriate for your Mac. Here is a site that compares best deal memory prices in the US:
http://www.ramseeker.com/memory/MacPro_Memory_Upgrades_DDR31333/
These companies offer a lifetime warranty. If it ever breaks, they will replace it, free. If your "maybe it works" memory does not work at any time in the future, you get to buy it again.
