That's a really hard question to answer without knowing what your needs are or what your budget is...
Certainly, the line in on the old mac pro is not the correct way to connect a guitar to your old mac pro - that is a massive impedance mismatch - and the line in port on most macs are usually quite noisy.
A decent, but relatively inexpensive USB solution is the Focusrite Scarlett 2i2 - you can find these just about anywhere online and even most likely at your local Best Buy or Apple Store.
For a little extra money, you can get a really nice interface like the Apogee Duet or Focusrite Forte that is geared towards singer/songwriter markets.
And then there is the new UAD Apollo Twin - which is both an audio interface as well as a DSP for the awesome UAD plugin suite.