What is the maximum memory limit in El Capitan?

The RAM limit is hardware based not software based. The RAM specifications posted by Apple are based on the RAM available when that Mac was sold. When RAM density increases, Apple does not upgrade the maximum RAM specifications.

The hardware limit for the Mac Pro Early 2009 appears to be 128 GBs.

http://eshop.macsales.com/shop/memory/Mac-Pro-Memory#1066-memory

OS X provides approximately 18 exabytes (18 million TB) of addressable memory for 64-bit processes. See:

https://developer.apple.com/library/ios/documentation/Performance/Conceptual/Man agingMemory/Articles/AboutMemory.html

C.

As cdhw says OS X can in theory address 18 exabytes of memory i.e. 2 to the power 64. However as Lanny says the official hardware limit is 128GB of memory and that is only if you have a dual CPU configuration of Mac Pro. (The fact you are referring to a X5690 chip implies you do have a dual CPU model.)

Now while Apple do have official hardware limits for the amount of supported memory it has in some cases been found that it is possible to fit more than those official limits however this typically has been older Macs which 'officially' might have had a limit of 2GB or 4GB or memory and actually turned out to be able to do 3GB or 8GB. See http://eshop.macsales.com/memory/maxram

I am not aware of anyone having tried to fit 32GB memory modules to a Mac Pro so the fact that the Intel chip supports more than this might be promising but is no guarantee. If you can get the memory on a sale or return basis you could try it out and let us know as I am sure we would all be interested.

Note: Even the Mac Pro 2013 officially has a limit of 64GB of memory but is known to be able to do 128GB. The 2013 Mac Pro is known to work with 32GB memory but uses a different logic board, different Intel chips and different specification RAM.