Operation Flashpoint - Dragon Rising.....never heard of it. 🙂
I think Jake is mistaken; you're talking about a Mac Pro, as in the tower, not a laptop. The limiting factor will be the graphics card, so whatever card you throw in there will make all the difference. The top 2 PCI-E slots are PCI-E 2.0 with a lane width of 16, the two slots are PCI-E 1.0 with a lane width of 4..
An 08 octo (or even quad) has *plenty* of processing power to support any modern game (that's at least slightly multi-core friendly).
So yeah, a kick-ass graphics card should do the job, especially if you're in Windows (which has better drivers).