I have a 1990 744 (sedan), and I have done a (mostly) complete stereo job on it.
For the front doors, I bought a pair of Boston Accoustics Pro series seperates. Now let me just tell you this... Volvo has very small holes in their doors for magnets that are on the back of (decent) speakers. Unless you want to go with 4 inch speakers (Very bad for ANY hope of midbass; you will really be annoyed) you will need to take some metal pliers, and cut straight slits 1.5 inches deep every two inches or so apart around the factory cutout. Then take some pliers, and bend the sections back into the door frame (Or out of it, but it will give you sharp edges to contend with). Bending metal is VERY common in aftermarket installs. If you don't want to do this, you can buy speakers of less quality, with smaller magnets, that will fit in the factory cutouts. But in my opinion, it would be a waste of money. For the rear fill, I bought a pair of Boston Accoustics RX57 coaxials, with the tweeters in the cone, and had to do the same metalwork. For the rear fill, you could go with smaller speakers than I did, and do no metalwork. The rear fill is just that; a background to your music that gives it depth. Most CD's are recorded in two channels anyway, so for a four channel system, most of your music is going to be comming from the left and right front speakers, with the same music comming from their respective counterparts in the rear. So I would get a pair of 5.25 inch coaxials with smaller magnets to fit in the rear doors. Talk to me if you want to go further with your install (amps/Head unit/DSP/Stiffening Caps/cabling/etc.)
-Dan