I am in the PC world for years. I shoot many many DV in past 10 years. Of course in sd 4:3 format. Just come across Final Cut Studio 2 which has a function to convert sd (4:3) movie to HD (16:9) format. Seems that it is very powerful.

Do you try this? How is the video quality?

Final Cut Studio is a very extensive package of programs. It contains:
*Final Cut Pro (professional editing system with support for many codecs)
*Motion (making animations)
*Color (professional color grading system that costed 25k, to 5k, to free in the studio package, although it has some bugs)
*Soundtrack Pro (editing the soundtrack, making a 5.1 surround track, etcetera)
*DVD Studio Pro (to design and make complete dvd's with menu's etcetera)
*COMPRESSOR (what you are talking about, for encoding from one format to another, you have very much options).

I think the package as a whole is very powerful, gives you a lot for you money, but compressor is only one of them. I wouldn't think it would be worth it to buy a complete Mac system for only the compressor program, but if you are looking for the complete studio package, then it's maybe worth upgrading to a mac.
You can set different quality settings, and I think quality is pretty good, although someone (if I remember correctly) made a thread about the differences in quality when encoding from HD to SD in an NLE. I don't know if Final Cut was mentioned too...

Thread is here:
http://www.dvinfo.net/conf/showthread.php?t=70792&highlight=hd+sd+conversion
EDIT: the thread only is about PC solutions.

Final Cut is becoming a pretty much used package, there are already 800.000 registred users out there, and we have our own board for it (Non-Lineair Editing on the Mac).

Best regards,