2004 S60R vs 2007 S60R?
 

What are the differences, if any, over the years w/the 2004 to 2007 S60Rs?

'05+ P2 cars have faster processing ECU.
'04-'05 auto's are 5 speed and tq limited in 1st and 2nd gear', '06-'07 have a 6speed auto that is not tq limited.. well it is but it's very, VERY slightly limited. I can't remember off the top of my head what it was.
'04's have black plastic molding on the car, '05+ have the molding painted to match the car.
'04's have slightly different seats (The front seat's side bolsters are closer together).
'04's have the older style tail lamps
'04-'04 have an aluminum charge air pipe that runs over the engine, '06-'07's charge air pipes are plastic.
The SUM programing in '04 was pretty stiff and gradually gets softer through the years.

Bottom line IMHO, if you're gonna get an auto, go with an '06-'07. If you're going with a manual, get an '05+ because of the updated styling and faster processing power of the car.