dual displays?

None, really. There's a Matrox solution that can split a VGA signal into multiple screens, but there's several problems with it:

1) It's VGA. That means lower picture quality, and you buy an adapter for your mini.
2) Expensive. Dualhead and TripleheadToGo, I think their names are, run $200-$300. Take that alone and you're already at the price of an iMac, with which you can do dual screens with DVI.
3) Non-native display. The Matrox box basically tricks your computer into thinking it has one big display instead of two or three smaller ones. As such, your menu bar will extend across all monitors, a "centered" dock will appear straddling the two monitors, and dialog boxes could also pop up straddling the two monitors.

It's a subpar solution that can be bested, for the same price or less, with an iMac+external display.

 

pretty much sums it up, it is REALLY expensve for what it is worth. Sorry.