Dual 27" Thunderbolt
You need the 2.5 Ghz Mac Mini, which indeed also comes with an AMD Radeon HD 6630M instead of the integrated Intel HD Graphics 3000 on the 2.3Ghz model.

For a list of models that support up to two Thunderbolt displays connected, check this support article over on the Apple website:

Thunderbolt ports and displays: Frequently asked questions (FAQ) 


14 - How many Apple Thunderbolt Displays can I use with my Mac in OS X?