I haven"t ever registered an account here, but I was reading this thread and wanted to dispel some misinformation.
I very recently bought a very similar Mac Pro to you (same model and processor speed, but 16gb ram) for $600, however I think it's worth $1000 (At most at those specs).
I couldn't say no for the price I got it for, but I have been putting the PC through it's paces.
This Mac would DESTROY even the latest Mac Mini when it comes to games, it is not even comparable. The dual Xeon processors when measured together are better than a i7-2600, the only difference being the lower single thread performance due to the old architecture. However the single thread performance is still better than you currently need for any game.
I installed an SSD and a GTX660 in the Mac Pro I got, and after benchmarking many games, I am getting equal FPS to other systems which are the latest i7's.
These are the pros and cons of getting a 2008 Mac Pro:
Pros:
- You can install modern hard drives and 3D cards, which you cannot do with Mac Mini's and iMacs. If you compare the latest Mini's and iMacs from now, and then in 2 years you install the latest SSD and graphic card, it will beat them in performance.
- The memory is expensive, but if you get over 10gb, it will most likely be enough for most applications outside of video.
- The 8 cores available to you are still above most modern consumer CPU's, and more and more new applications and games are using multiple cores. As soon as an application uses all 8 cores, the importance of single thread performance decreases.
Cons:
- CPU's are the top range of the old CPU architecture, one CPU is equal to like an i3-2100, and the single thread performance is lower than that.
- The ram is EXPENSIVE, you will pay $120 for 8gb more ram.
- All the techs used are old. No Thunderbolt, no USB 3.0, DDR2 memory, no PCIe 3.0, IDE for CD Drives. My argument for this is, unless you want a thunderbolt display, the port is pointless, USB 3 is cool, but not really that important unless your work entails copying huge amounts of data over it, DDR2 is only like 5% slower than DDR3, We are hardly even reaching the capacity of PCIe x8, let alone x16 or more.
- Possible lack of OS support from apple in the future
- No CPU upgrade options
Oh yeah, and you can SLI in windows, I'm planning on getting another 660 GTX, then I will get better game performance than 99% of the pc's out there

And definitely any Mac Mini or iMac.