Oooh, it seems to me you need a lot of information. I'll try to deliver the basics. The transport reads the digital signal off the disc. This signal is in the form of numbers. ( The numbers happen to be in binary format because we have no better way of encoding them at the moment. )
The converter understands what the numbers mean in terms of music. It converts them ( as you would expect from the name ) into an electrical wave form which resembles the original acoustic wave form of the music. This can be amplified and sent to speakers. A digital signal, without conversion, would sound horrible.
In other words, with a transport but without a converter, you have information but no music. Your present CDP3 has a converter in it but not all converters do the job equally well, and apparently the CDP3's performance can be beaten by an outboard unit. ( You would connect the CDP3's digital output to the new converter's input. )
That you should bypass your transport's internal converter and send the digital signal to an external unit is not an uncommon phenomenon. For example, Linn's Karik transport had an internal DAC of basic quality. You bought the Karik first, then added the external Numerik DAC as an upgrade.
I can't tell you if you should get into SACD at the moment. My own choice is negative, but I already have a good high resolution source in the form of a vinyl playback system. If I did not have this, I would be planning on adding SACD capability eventually.
I haven't heard the Proceed converter. I have an Apogee DA-1000E.