I just did that little task this past weekend on my wife's 2005 S80. I used the following document as guidance and it was fully applicable to the S80, so I assume it would be a big help on your S80.

http://www.v70xc.com/0/resources/how-to/pdf/heat_exchanger_removing.pdf

The only thing I did in addition was to flush the system before removing the old heater core.

Once you disconnect the hoses on the engine side of the firewall, you can take a piece of scrap hose slipped onto of the nipples and blow through it. It will push the majority of the coolant out of the core, making for less of a coolant spillage in the drivers footwell when you pull the core out.

Also, at least in the S80, it's impossible to R&R the core without bending up some of the cooling fins due to the angle at which you have to tilt the core going in and out.

I went the the cheapest of cheap heater cores from Advance Auto. Time will tell.....