It would seem that hard drive speed is a issue, if you are saving the mp3 to the computer.

Get a quad-core processor if the application is "core-aware". Fastest CD is a plus but you do need a good beefy CPU too if you want to do (encode it to Mp3) it quickly. Do not wimp out on the CPU please.

I highly doubt that the app is multithreaded, so any "modern" single core CPU will work perfectly and not break a sweat (if it's not multithreaded and the machine is only being used for ripping there won't be anything else running on the machine to use resources. There's no need to worry about the CPU, don't waste money upgrading it beyond a standard 2.4 (IMHO).

only if the app is multithreaded, and considering that it was a custom app, i highly doubt it's multithreaded, so don't waste the money on a dual or quad core cpu, a standard 2.4 with 2GB of RAM will be more than sufficient imo.

Might as well go with a dual core cpu since are pretty standard these days, it usualy costs the same or not much more than single core legacy cpu's but I agree 2.4 with 2gb ram is plenty