Augusto: I don't know if you purchased the Mac Pro 2009 already?
If you did you can potentially double it's speed for about 80 Euros, which in my opinion it's pretty close to the performance of the i7.
My current setup is a Mac Pro 2010 I purchased refurbished from Apple last year.
Just over a week ago I became interested in PC Games after some disappointment in the new consoles (Xbox One) and saw that I could do better with my Mac Pro.
I had an original Apple branded ATI Radeon 5770 HD Graphics Card, I have put in a PC card in place of that which is a GTX 660 by EVGA, which has already gone over double in performance there alone.
My next upgrade is the CPU...
I have the 2010 Mac Pro with just one Quad Core (4x) CPU running at 2.8Ghz
My options were getting the fastest compatible chip but that costs too much for me at the moment, this was a 3.43 Hex Core (6x) for 200Euro's
After looking around and asking on here, I found out that I could install a smaller hex core Xeon X5650 chip which cost me 60Euro's shipped.
When I install this chip it will make my system twice as fast, which is crazy enough and enough power for me for what I do and plan to do.
If you haven't already purchased a Mac Pro system, perhaps look for a 2010 model since you wouldn't need to flash the bios and the 2010 are only about 30% more than a 2009.
If you can spend abit more, get a Dual Quad system where you could install 2 cheaper chips and tripple the speed.
My current 2010 model benchmarks at 8554 with Geekbench
When I upgrade the CPU it will be twice as much as that.
If in the future I have some extra funds, I plan to get a Quad CPU Board for my 2010 and I'll install 2 of the 2.66 Hex Core X5650 and I'll get over tripple my original system.
I hope that makes sense for you in Italy.
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P.S. Also the 2010 is easier to upgrade the CPU since it has a better socket design on the CPU chip holding mech.