Can not upgrade to Mavericks from v.10.6.8 Snow Leopard
Please help me understand why can I not upgrade to Mavericks from my v.10.6.8 Snow Leopard. My MacPro2,1 machine is of late 2008 (I think, but I can not verify the year), she has a 3 GHz Quad-Core Xeon chip, 8 GB of 667 MHz memory and 600+ GB free space on the disk. The Apple upgrade page says that "OS X Mavericks cannot be installed on this computer", accompanied by a "Learn More" and an "OK" button.
Mac Pro, Mac OS X (10.6.8), upgrading the OS