Okay. But how does the BTO 2.7 GHz i7 Mini perform? Or better yet: how does it compare to my 2x2.26GHz Mac Pro? I believe it takes 8GB of RAM. Will that be enough?
As far as memory and CPU are concerned, a stock i7 Mini (2.7 GHz, two cores, four threads, 4GB RAM) gets a Geekbench score of about 7760. The Mini Server (2.0 Ghz, four cores, eight threads, 4GB) gets about 9460. A 2009 2.66 GHz Mac Pro (four cores, eight threads, 3GB RAM) gets about 9240. Yours would be lower than 9000 with only one of your CPUs running because of the significantly slower clock speed.
ECC is meaningless in photo editing. Errors happen all the time, and ECC can't fix most of them anyway. Google and University of Toronto did a study on it.
If you're running a render farm or something and precision and uptime matter, ECC is worth the additional cost. For everyone else, it's just a waste of money.