Yes, the Xeons that are in the Mac Pro are the '5100 sequence', to use Intel's terminology. 50xx and 70xx processors are based on the older NetBurst architecture. Although I do wonder if they would work in the Mac Pro? The chipset supports them, after all. Which means if you put in two dual-core 5080s, you'd have four physical 3.73 GHz processors that each have hyperthreading turned on, making the OS think you had eight processors... Of course, that would require 100W more watts of power than the 3.0 GHz 5160 that is optional on the Mac Pro... (130W each as opposed to the 3.0's 80W, and the stock 2.66 GHz 65W.)
P.S. It's the 7100 series that is 'multi-processor' capable, the 5000 series is only dual processor. (That means only two sockets per motherboard, where 'multi-processor' means more than two sockets per motherboard. Dual-core processors only count as one socket.)