Non-Mach amp/speaker upgrade - stupid or worth it?

Basically I've got a '00 GT with the 2 piece stereo that looks like a mach 460 but it isn't, doesn't have the door tweeters or anything, only 4 speakers.

I'm looking to retain the factory head unit and upgrade the amp and speakers, is the head unit that terrible?

My main motivation in retaining the factory head unit is to have a better sounding system, but not attract thieves, they rip stereo stuff off way to often around town.

Has anybody done this? I've got an aftermarket 4 channel amp and another 2 channel so all I'd need to buy would be speakers, hopefully.

Thanks!

Yeah, theft is a big deal to me, nothing would piss me off more than coming out to find a broken window or pryed out one. On the ipod issue, I simply use a tape to headphone adapter now to hear my mp3 players through the radio, it seems to work really well!

I looked at this article, http://mustangworld.com/ourpics/fcar/despeeds1.htm but my non-mach unit doesn't seem to have one of the connectors referenced in the article. I suppose it would be simple enough to just use the factory speaker wires for the input to a line out converter but I was thinking there may be a cleaner/better way.

I read somewhere that the factory head unit has loudness permanently enabled, or something like that. The unit appears however, to be identical to the mach unit just with a different amp and speaker configuration. I wonder if the part numbers for the head unit are the same on mine and the mach unit?

Thanks!