interface
Considering you have the preamp you prefer, all you need is an interface that can deal with line-level signals. But then again it's true that the sky is the limit. Or you could do it dirt-cheap. I doubt your hi-fi set-up can give you the +4dB levels standard in most pro-equipment, but you will have no need for that provided cables between your preamp and the interface are kept reasonably short. I do not personally have experience with the apogee duet, but I heard great things about it and the Apogee stuff is great quality built for (and in a way by) Apple. It does seem to fit your bill. That is, if you would use your G5. (I certainly would). As for software, almost anything would do. Even Garageband. Or Logic Express. Or Audacity. Unless you plan to do dynamics editing. Of course, if your final target is mp3 or some other compressed format, all of this is perhaps a bit over the top. From your gear, I just kinda guess you don't care for mp3s, I could be wrong? Good luck!
By the way, Sesshi speaks of the RME Fireface, which is an excellent product (He does not say if he has a Ff400 or Ff800). I use the Ff400 with an iMac myself, but it's a much larger interface that I use for music production. (More simultaneous channels in and out of the computer). If only converting vinyl to digital, I would say it's excessive.
