Considering that your needs appear to be rather basic, could you not simply get an adapter and use the 3.5mm input jack on the Mac Pro?
Link: GOLD-PLATED TWO PHONO JACKS TO 3.5MM (1/8") STEREO PHONE PLUG | Cable Adapters | TheSource.ca
I don't have much experience with audio input cards, but unless you're looking for multi-channel, it may not be required.
Luc
Link: GOLD-PLATED TWO PHONO JACKS TO 3.5MM (1/8") STEREO PHONE PLUG | Cable Adapters | TheSource.ca
I don't have much experience with audio input cards, but unless you're looking for multi-channel, it may not be required.
Luc