There is a lot of super heavy shader work there so most likely some beefy GPU running in sli-mode or bridged (depending on the card) because I'm thinking they have to be running this thing on at least 2 video cards...
On 2nd thought, maybe not. I'm sure just one card aimed at gaming enthusiasts will do the job. I mean the entire engine (unity) was designed for real-time scenes to be used so just a single card would probably do this if it was "good" enough.