Many vendors sell ipod adapters for the factory radio.
Or, you can put any radio in it that you want with the right adapters. Volvo makes a factory double DIN mount, available from Tasca here. Click "view illustration" to see a pic of it, and buy the pocket too if you get a single DIN. It's easy to install and looks great. I just put one in my '01.
If you have a factory amp you might have to have it deleted through VIDA so the car doesn't try to use it. Also, with an aftermarket radio you lose your steering wheel buttons.