Relative newbie to the digital audio world here, so be gentle please.

I want to get cassette tapes of original music that I recorded years ago
onto CD. In the process, I'd like to do some minor tweaking (e.g., roll off
that 50 Hz bass sound, edit out that unnecessary chorus, etc).

After talking to several people and doing my homework, my current plans are
to get a Powerwave USB audio Interface for the A/D into the machine (there's
no audio in on my machine), then use Toast w/ Jam to write the CDs, and
possible get a equalizer plug-in to do the tweaking.

What I'm looking for is a reality check here. While I don't need absolute
professional recording studio quality, I'd like to do as best as I can here.
For example, if there is A/D USB input that would give me superior quality
for, say, another $50, I could go for that.

Also, are there any "gotchas" that will seriously improve/degrade quality?
E.g., I saw a thread somewhere that spoke about having to keep the input
level in the middle on your System Preferences.

I'm using a 733 G4 Mac about 2 years old w/ 768M, running 10.2.8. Thanks in
advance for your advice.