Hi there,
I think most people will tell you to try to find a 4,1 minimum. Honestly, as the owner of a 3,1 and a 1,1 Mac Pro, I will verify that recommendation. The 3,1 and lower use DDR2 FB-DIMMs which can get expensive and are slow, and have a limited Front Side Bus architecture. The 4,1/5,1 machines are a bit more upgradable. If you're going to save for a Mac Pro tower, get a 4,1 or 5,1. Unless of course, you get a steal on a lower model. I grabbed my 3,1 and a 1080p Acer monitor for $200, and my 1,1 "non-functioning" for $70 and gas.
As far as gaming goes, it depends on the game you are playing. My Dual Quad 2.8GHz 3,1 runs BioShock Infinite at 60FPS with my GTX 660 2GB SC, however it only uses 2 cores of the available 8. I've found that the Xeons in my 3,1 are about comparable to a Q9550 Core 2 Quad, if you're looking to compare models there. The 4,1 and 5,1 use a different socket, but the higher end i7 CPUs from that generation (Nahalem architecture) with similar clock speeds. In my experience, if you have the GPU power to run the game, you will be happy at 1080p at a reasonable frame rate, especially on the 4,1+ models.
Keep in mind, the 4,1s can be upgraded to 5,1 processors with a firmware hack for future upgradability. There is a thread in this forum
HERE that is very useful for processor upgrading instructions.
tl;dr - get a 4,1 or a 5,1 unless you come across a really cheap 3,1. Don't go near a 1,1 or 2,1. Gaming is pretty decent with a new modern GPU.