It isn't financially sound to put a $400+ 2.16GHz C2D in a Mini right now IMHO, unless you have the CPU laying around or get a crazy good price somewhere. Even the Yonahs are still pretty pricey.
You can get a 1.66 C2D for around 200 bucks, but I doubt that will make a huge difference over your current CPU.
Still, when the price does drop, it will be a very nice upgrade. Ever since they went Intel, the upgrade potential of the Minis jumped quite a bit. And having two DIMM sockets rocks. Just wait till 2GB DIMMs get cheap...