Look in the Audio forum. There is a great post about installation of an aftermarket double din deck. I believe it's the kenwood, which is 5.1 so you can use the center speaker. I was going to go with the eclipse with the built in nav, nice unit BTW.
Volvo also makes a double din adapter and a single din adapter. I bought the double din and its perfect. However, I've decided not to install the double din deck.
I went with an aftermaket system using the factory deck, alpine amps, a 12" sub and a digital EQ specifically designed for factory decks.
It's a great set up while maintaining the factory appearance. I just felt that my money could be better spent on other things. Plus my wife had a lot to do with it.
Good luck.