Have early 2008 Mac Pro but can't install Yosemite???
Have an early 2008 model Mac Pro but App Store says it won't install.
MacBook Pro, Mac OS X (10.6.8)
You don't have a 2008 model. You have a 2007 model you bought in early 2008.
Upgrading to Lion
If your computer does not meet the requirements to install Yosemite, it may still meet the requirements to install Lion.
You can purchase Lion at the Online Apple Store. The cost is $19.99 (as it was before) plus tax. It's a download. You will get an email containing a redemption code that you then use at the Mac App Store to download Lion. Save a copy of that installer to your Downloads folder because the installer deletes itself at the end of the installation.
Lion System Requirements
1. Mac computer with an Intel Core 2 Duo, Core i3, Core i5, Core i7,
or Xeon processor
2. 2GB of memory
3. OS X v10.6.6 or later (v10.6.8 recommended)
4. 7GB of available space
5. Some features require an Apple ID; terms apply.