Check external soundcards that connect to your mini via Firewire (or USB). I am partial to Firewire connections myself. There are lots of interfaces that will do nicely, just make a list of what you would like to be able to input and output from your mini, and then go see what offers you this at the best price. You should be able to get this information from many of the Pro-Audio webstores like Sweetwater.com, etc.
I use a MOTU 2408, but this is an older interface that uses a Audiowire connector that has its own PCI card in my G5. MOTU have other interfaces, the "Traveler" is one, that use Firewire connections. These might be a little too overboard for what you want to do. The Traveler offers a lot of input choices and output choices and is great for live recording using Mics, Line inputs, Digital inputs, or analog inputs.