Do it yourself

I moved into my first house a few months ago. The house was built in '86 and it is very basic style (I believe defined as 'tract home' style) with no interior trim around the windows or doors, no ceiling moulding, and only the most basic base boards. One of my first improvment projects I want to under take is to trim one room with crown moulding, base boards, and finish work around the doors and windows. So I went to the library and found an article in Fine Homebuilding magazine about trimming windows. The article was more about trimming windows in new home construction, but it still gave me some great ideas. But the article talked about installing "jamb extensions" inside the rough opening of the window. The windows in my house are sliding windows in aluminum tracks, and the "jamb" is already covered in drywall. So here is my question: if I want to install wood trim around the inside of the window opening, should I rip out the drywall first, and attach the trim to the framing? Or should I just glue and tack the wood trim on top of the drywall and cover it up? Any opinions or knowledge about this? thanks, Colin