I went with one of those, also.
http://www.casmfg.com/Secretaudio.htm

I liked the flexibility of it vs converting the stock radio to modern internals. The converted original radio still wouldn't have worked with the planned amps, subs, etc in my car.
The SST setup has mp3 & iPod jacks, CD changer, & Sirius. Can hook up external power amps.
The receiver can be hidden anywhere, and the faceplate is low profile so you can hide it easily. They say you can put it recessed in your sunvisor. I hid mine inside the console. Comes with a small bracket in case you want to mount it under the dash. Stock AM thumbwheel dummy radio is in the dash.
Comes with a small remote, too.

I have a stock AM radio/modern conversion in another car. Nothing elaborate. Dual voice speaker in the front with 6x9's out back. Conversion was performed by this company:
http://www.garytayman.com/