Be careful with Napster. As you've said, you have to continue your subscription to keep the music. As much as it hurts me to recommend an apple service (shuttup grogie!) I'd suggest looking into Itunes. The files come over in a non-mp3 format, but you can burn them to a CD and then rip the CD to MP3 very easily.
For aftermarket radios, if you're interested in doing aux inputs, etc, I'd look into Alpine units. There are a good number of adapters available for alpine, and your options are reasonable with them. As an example, Crutchfield has this unit for $150 - if you're interested, I can get you a coupon for $10 off and free shipping if you use a Visa card.
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Alpine CDE-9843 CD/MP3 receiver
Crutchfield is great, they include all the stuff you need to install it (the faceplate adapter, a factory harness adapter, and complete instructions on how to get the old radio out, etc. I'd highly recommend them.
I have an Aiwa in my car, which has a 1/8" jack in the front. While I'm happy enough with it, I really wish that the aux input was in the back. Makes for a cleaner install. Having a player that plays MP3s was pretty cool (before I got XM) - it works pretty well. For me, being able to burn MP3 CDs was a very cool option - that way I could have as many mixes as I wanted in the car with me, w/o having to mess with it. We hardly use it now that we have XM, but it's still there
-Blu