98 Forrester overheats

I'm looking into this Forrester that overheats.The thermostate has been changed and ran fine for part of the day.Went to head out of town and only got a few miles out of town and it overheated.I suspect a head gasket has failed.I would like assistance to determine which side is the culprit.Would it be just wise to change both sides??I'm also looking to find info on the repair proceedure.I would appreciate any help.The vehicle has a 2.4 litre motor with standard tranny.
chuck's

if u think ucan handle it. its not a novice job just so u no. i would change both while ur there.
SAMSUNG-SGH-I617/1.0 Mozilla/4.0 (compatible; MSIE 6.0; Windows CE; IEMobile 7.6)

I did both head gaskets.It was a time consuming job,taking it slow and easy paid off.Things were a little tight,especially the left side,but was able to manuver the heads out.I didn't even curse the engineers.It must be a challenge to change spark plugs!!The timing belt went on easy also.
chuck's