There are specs or my 2008 Mac Pro running OS X Yosemite 

 

2 x 2.8 GHz Quad Core Intel Xeon

16GB 667MHz DDR2 FB-DIMM

Nvida GeForce 8800 GT 512mb

 

So I was wondering if my Mac Pro would run FSX with the PMDG 777 at 20+fps at descent settings using windows 7 home bootcamp. Thanks!

Probably not I think you will have compatibility and performance issues.

No, not at all. Need to be in the 4.4 to 4.6 GHz range. 2.8 is not even close.

How about just FSX alone?

It will run, but very basically with low level settings. If I were you, I would look up the AVSIM guide. It'll provide instructions on hardware and tuning-specifically tailored for FSX.