I have a 1st Gen Mac Pro and I use it for WoW and I also have L4D2 (Steam). My only thought are, avoid it like plauge!!!
Since it have 32-bit EFi you are stuck in the past and the only thing you will achive are that you have paid alot of money for the Mac Pro and for upgrades which achives nothing. It is still a 4 year old slow machine (for gaming) and nothing can change that.
In theory this a upgradeable machine, but not in real life....
If I had the money (and Apple had another choice for gamers) I would have replaced my Mac Pro 2-years ago because it was allready old back then...now I am stuck with this one until....I don't know...Steve decides to sell us a flexible/upgradeable i5/i7 offering which probably never will happend or I buy a gaming computer