Don't confuse power (which is measured in Watts) with sound quality: a system might have 600 Watts, but that doesn't guarantee it'll sound good.
Many manufacturers offer rear speakers only as an option, so check the supplied speaker fit before you sign on the dotted line. Often, adding rear speakers is surprisingly cheap, so long as you ask when you order your car and not when you've got it home.
Look out for an MP3 player input, which allows you to link up any type of personal music player easily, including iPods.
Better still is a specialist iPod preparation, which will give you full, safe control over your listening using your car's standard hi-fi controls.