Also attached an eGPU GTX 770 4 GB setup to my Mac Mini 2012. Selected the GTX 770 4 GB because of compatibility with OS X Mountain Lion (it's a better GTX 680, same GK 104 chip).
Parts:
Thunderbolt to PCI Express Card:
http://www.sonnettech.com/product/echoexpresscard34thunderbolt.html
PCI-Express Card to PCI Express (PE4L v2.1 supports PCI E 2.0):
http://www.hwtools.net/Adapter/PE4L V2.1.html
The case is a small ATX case, Cooler Master Elite 130:
http://www.hard-planet.net/wp-content/uploads/2013/06/14.jpg My decision for this case was because you can just install all the parts with minimum effort, including an ATX PSU.
Editing the Nvidia drivers and OS X KEXT's in Mountain Lion and Mavericks is really easy. Not yet tried in Yosemite.
The Mac Mini is on the table, the eGPU under the table, i am using a 2m Thunderbolt cable.
Some Unigine Heaven benchmarks (Opel GL 3.2, OS X Mountain Lion)
Mac Mini 2012 iGPU Intel HD 4000:
Setting Basic (1280*720, x2 AA): 11.7 fps, Score 294
Setting Extreme (1600*900, x8 AA): 3.3 fps, Score 84
Mac Mini 2012 eGPU GTX 770:
Setting Basic (1280*720, x2 AA): 65.3 fps, Score 1645 ->
+ ~559% compared with Intel HD 4000
Setting Extreme (1600*900, x8 AA): 41.7 fps, Score 1050 ->
+ ~1250% compared with Intel HD 4000
Works flawlessly! Including playing HD movies from iTunes Store, DVDs, CUDA acceleration etc. And that's the difference to a Hackintosh; you still have a normal Mac, but with a decent GPU.