Unfortunately it looks like that model is only compatible if you upgrade the CPU first.
Probably not worth the trouble at this point. You could still add some RAM and an SSD if you wanted to speed things up, but it won’t go beyond OS X 10.10.
Machines that ARE NOT supported:
-2006-2007 Mac Pros, iMacs, MacBook Pros, and Mac Minis (MacPro 1,1 and 2,1, iMac 4,1, 5,1, 5,2, 6,1 and 7,1, MacBook Pro 1,1, 2,1, and 3,1, Macmini 1,1 and 2,1)
*The 2007 iMac 7,1 is compatible if the CPU is upgraded to a Penryn-based Core 2 Duo, such as a T9300.