Quad Core
I recently upgraded from a 2007 iMac to the latest Mini... wanted to bring my own monitor and didn't want to wait for the iMac's any longer.
I do a lot of work in LR 4 and the system I put together works fantastic...
2012 MacMini with Quad Core i7
16Gb RAM (self upgraded from OWC)
256Gb SSD (factory)
4.0Tb External USB 3.0 "MiniStack" from OWC
ViewSonic VP2770-LED 27" monitor
With the OS, LR 4, and the Nik plugins on the SSD and my pictures on the External USB, the system flies... I find the performance of the USB 3 external drive equally as good the internal I had on my iMac, which was sufficient. I know one of the knocks on USB is the CPU load it introduces, but I experience no lag because of that... the CPU has plenty to give!
Hope this helps in your decision making process...
--Rob.