A good starting point for information would be
www.irrigationtutorials.com ... specifically the page
http://www.irrigationtutorials.com/dripguide.htm
The guy at hunter was somewhat correct. With a drip system, you have to make sure the pressure has been regulated down to about 30-40psi, and the water has to be finely filtered. Without a filter (even on city water), the drip emmitters will quickly become clogged.
Now if you had something like the RainBird 1804 pop-up bodies, you could use something like this:
http://www.sprinklerwarehouse.com/1800-RETRO-Retrofit-Kit-p/1800-retro.htm It's designed to replace the spray head. You then just need something to transition from threaded to the 1/2" drip tubing. Howeve, you will have to repalce one of the sprays with this retro kit (and likely also purchase a Rainbird 1804 sprinker head for the retro kit to fit in) and cap off the other two sprays, or convert all three and have the water coming into the garden from three different directions. The other option is to rip out the existing spray heads, reuse the pipe, but you'll have to add the filter and pressure regulator before the valve. So you might have to rip out the valve as well. Likely the simplest solution is to get three of these retro kits and (likely 3 1804 Rainbird sprinklers to put the retro kit in) and just replace the existing spray heads. However, that will only work if these spray heads only feed the garden. If they feed other spray heads on the lawn, then its going to be difficult (not impossible, but difficult, even for a professional) to match precipitaion rates in the drip system AND the lawn sprays on the same circuit. A drip irrigation system needs it's own circuit.