I've got the Pioneer DEH-P8000 and 6815's all around, with a Pioneer 12" sealed sub in the trunk, 200 watts powering that. Insane, and relatively cheap (under $1000 including installation, thanks to the net prices).
If you're looking to save some cash, leave the rear speakers alone, only your rear seat passengers will really notice the difference. Wish I'd done that and saved the money, but I'm still really happy with everything.
