If the named computer is a Mac Pro (tower) then it should be able to use up to
32GB of RAM installed across several memory upgrade slots, and also have
no issue running OS X 10.10 Yosemite. An item of historical note for that model
is there had been a Firmware Update for it some time ago, that may have been
required to be installed prior to later OS X versions and support function.
•Mac Pro (Early 2008)
Introduced January 2008
Discontinued March 2009
Model Identifier MacPro3,1
Model Number A1186
EMC 2180
Order Number MA970LL/A (two 2.8 GHz)
Maximum Memory: 32 GB (none affixed permanent to logic board)
Memory Slots: 8 - 240-pin PC2-6400 (800MHz) DDR2 ECC fully-buffered DIMM (FB-DIMM) (matched pairs)
[from http://mactracker.ca download database app for model info given]
•However, if the computer is a notebook model MacBook/Pro, that is quite a different
animal and requires slightly adjusted care and feeding intervals. And an early model
may not be able to run Yosemite if the hardware isn't capable. Mountain Lion?
Please supply more build model. You may have to reset SMC and perhaps NVRAM
to be sure the computer is OK; and perhaps perform a hardware test.
Good luck & happy computing! 🙂