The thing is that I want a mac desktop but not the iMac, so it's between Mac Pro or a Mac mini.
Do you want a Mac desktop for a 'Mac desktop,' or do you want a Mac desktop for Mac OS X?
The *only* reason one should even really consider a Mac for a solely gaming machine is on the requirement for Mac OS X alone (IMHO), and even with that, it's a (rather) weak one.
OpenGL support is far more up-to-date on the Windows and Linux side anyways. With that in mind, you'll already be taking performance hits playing cross-platform games within Mac OS X.
Yes, a Mac Pro
can be a nice gaming machine. But honestly, unless you're using it like a work-horse (as it's designed to be) on Mac-dependent software, I cannot fathom why someone would be willing to pay >$3k for a gaming machine that could be built in a Win/*nix environment for less than half that.
I truly do believe this is a fool's errand you're chasing. If you
absolutely had to choose based on a 'need' for Mac OS X, I would steer you first to an iMac, and secondly to a Mac Mini. But even those are with hesitation. After owning my Mac Pro for the past 4 years, and even attempting to bring it up to speed with a GTX470, I would still opt to build my own Win7 gaming box should I finally find the time and/or games to sit down and play on a semi-regular basis before considering a Mac system to do it.
EDIT: In order to answer your original question (so I don't feel as if I am utterly soapboxing here), unless someone can correct me otherwise (been a while since I've built a machine in the multi-core era), if you're building a 'serious' gaming rig: a quadcore (HT) processor at the highest clock you can attain, at least 8GB of RAM (16 is likely ideal), and obviously the best (non-workstation) graphics card you can attain, with an adequately-sized SSD to host the OS and game installations. But these are pretty standard for any gaming system build, IIRC.