I'm an OS X only gamer but 've actually found my 09 Mac Pro has done well over the years despite the low 2.26GHz base clocks.
I've changed the GPU over the years from the BTO 4870 to a 5870 and now a 7950 going up to a 1440p screen in the process and it was all made more affordable by being able to sell the older GPUS on.
Although I did a lot of H264 encoding over the years which the cores really helped with (especially when I wanted to do something else as well) and audio work in Logic X etc. I really got this over specced machine simply because I'd enjoyed owning and expanding a sawtooth G4 and Power Mac G5 before it.
The new Mac Pro despite the nice new design I've concluded isn't really an option for me now given the change in focus to Open CL and video/rendering plus the price (especially as only the D700's would be better than the 7950 in OS X games) but I'm not sure what the next step should be....
On the one hand the Retina iMac when upgraded to the best CPU and GPU has massive single thread performance and just beats the multicore performance I have and the mobile 295X is slightly better than my 7950.
On the other hand Xeon CPU upgrade options for this machine including the possibility of dual hexacores if I flash the firmware are getting more affordable and that should give me better single thread performance closer at least to modern machines with truly massive multicore and it may yet be possible to get a worthwhile GPU upgrade again down the line although this will not likely be a "Mac edition" now and I could do with that boot screen...