Basically, the higher the wattage you have for your plants, the more you need to add CO2. Whiskers is right, you should read up on the subject though....there's a lot of related aricles out there that will help to understand it all.
If you want a low tech planted tank and would rather not deal with the whole pressurized CO2, I'd suggest go with plants that do well with lower wattage lights.
When I upgraded my tanks lights to 65 watts (>3 watts per gallon) I didn't realize this meant I also needed to add CO2. I've gotten by with the Hagen manual CO2 kit, but my plants would probably be doing better if I went the pressurized CO2 route.