I've been shopping for a NAV unit for my '06. Bought a REC from ebay, but it was DOA. Still waiting for a refund from seller, but that's another saga... I went shopping and looked at Pioneer AVIC D3 which is double din. Not sure if it will fit '04. 5 or not. Going aftermarket can be a compromise... I was told by Crutchfield that I would lose my factory Sirius tuner (and 1yr free service) and I'd have to buy a Pioneer tuner and buy a subscription. You will lose your steering wheel audio controls (if '04. 5's have them). An aftermarket module is available that accepts steering wheel controls and converts them into something compatable with Pioneer, but, from what I saw, you have to do a bunch of splicing which I'm not ready to do on my brandy new '06. So, even though Pioneer has Blue tooth, touch screen, larger display, and can play DVD movies, it's $1K w/o extra modules etc, and I don't want to lose my steering wheel controls (or hack into my wire harness), I don't want to lose my Sirus subscription or have to buy another tuner, and I don't want to make my truck more of a break-in target, so I think I'll stick with factory REC NAV.
Hope this helps...