Hi dr,
I use an RME Fireface (firewire, up to 24/192) to Logic Studio and my son uses an M-Audio PreUSB (~$100, up to 24/96) to Garageband. Both have line, instrument, and mic inputs. The M-Audio is a little noisy, but you get what you pay for. The line input on a Mac is just that: a line input, and cannot be converted to a high-impedance instrument input. Check out any of the online pro audio stores (Sam Ash, Sweet Water, Zzounds, etc) and search for computer interfaces. There are tons of them out there.
As an aside, the Apogee Duet, supposedly a great dac, also doubles as an instrument interface for Macs.