Installing Yosemite on Mac Pro Early 2008

I seem to be having a problem installing Yosemite on my Mac Pro, 2008. This computer is running Snow Leopard 10.6.8, fully up to date. I checked Apple's website to check compatibility and I am understanding my machine is compatible. When I go onto the app store, I get a window saying "OS X 10.10 is not compatible with this system."

The system contains:

2 x 2.66 Dual Core Intel Xeon Processor (Quad Core)

8 GB Ram DDR2 ECC (Does it need to be DDR3?)

Bluetooth 2.1 + EDR

NVIDIA GeForce 120 512 GDDR3

750GB HDD

10.6.8 Snow Leopard

I own a late 2013 MacBook Pro Retina (15" i7 2.3ghz, GeForce 750) and I connected my laptop to the Mac Pro VIA FireWire 800 (used thunderbolt adapter) and set the Mac Pro into "Target Disk Mode" and used the Yosemite installer and installed it to the HDD on the Mac Pro, but when I booted up the Mac Pro (after it installed), I got the blinking folder of death (Instead of Apple logo, flashing folder with question mark). At this point I had to use the Snow Leopard install disk, and start the Mac Pro using the disk, wipe the HDD and reinstall Snow Leopard and go back to phase one.

Please let me know what I can do.

Thanks!