As for the aftermarket head unit replacment and keeping the chimes.. there is a unit out there by Metra and I believe at least one other company to keep the chimes. Cost is around $60... Metra part number GMRC-01. Crutchfield and I think CircuitCity can get them (among your favorite online aftermarket stereo company).

I had one in a previous car. All the box does in plug inbetween the head unit and the factory wiring to "hear" the Data Bus signal. The box (which contains a speaker) then makes the correct sound for the signal. Problem is the box is buried inside your dash and can be harder to hear, or much louder and is no longer volume controlable. I know my Olds Alero I could control the volume of the "dings" with the stereo... I'm guessing you can still do the same.

Data Bus connections are standard on the radios... remember, it's a worldwide radio system now, so don't get too nervous if something doesn't like Solstice on the packaging just yet.