Frankly, if gaming is your top priority, you'll be served a lot better by building a Windows rig that you can build for a lot cheaper than buying a Mac Pro..
^^^ best advice
Not only that, there are 30 Mac Pro original configurations to date.
No one here has any idea which one of those you are referring to.
Not only that, but the CPU in the Mac Pro typically comes in at about $1,000 - $1,500 when new.
Ex. The current gen Mac Pro low end has two CPUs that are being sold at over $1,000 each and the high end (non-server version) has two that are about $1,450 each - those prices at newegg.
The lowest end CPU ever sold in a Mac Pro - the 2.0Ghz Xeon is still being sold (if you can find it) for just under $600 - due to rarity in finding one now.
Personally, doubt that you're going to "upgrade" the existing CPU(s) in any Mac Pro for under $500. You might "replace" a single CPU in a Mac Pro for that.