I bought my latest mini this past Spring. Introduced last March at the same $599/$799 starting prices, those models have offered excellent performance, a 5th USB Port (I mention this as a confirmed and unrepentant USBaholic), and built in dual monitor support. (You do have to buy a second, mini Display cable to use that second monitor. I got mine from an Apple Store for about 30 bucks.)
At that time, I stuck with the 2.0GHz processor and opted for the 4GB RAM upgrade and larger (320GB) disk drive. And, let me tell you, those choices (for which I thank all the people at Twitter who offered advice) were excellent. I haven't missed the top processor speed (there was a 2.26GHz option), the extra RAM was awesome, and I didn't have to crack the case to upgrade to a better disk. I recommend you do the same today: cheapest processor (now the 2.26 GHz chip), 4GB RAM, and the larger (500 GB) disk option. If you're in the mood for extra storage yumminess, consider the dual disk server configuration: an entire Terabyte of disk in a single sweet mini.
You can read more about this upgrade at Apple's refreshed Mini features page. And yes, the store is still down at the time this post went live. Maybe someone forgot to feed the hamsters.