seedlings were burning under 400w conversion- now wondering if 120w is enough

You gotta give us more info... 120Watts of what kind of light? Fluorescent lights have to be very close to the plants.. H.I.D lights have more penitration. Perhaps you need more ventilation.. Seedlings dont need very much light, but it would benifit your plants later on in their veg cycle if they got the 400watts of metal hailde. With the M.H bulb it doesn't have to be real close.. 2 feet away or so for seedlings.. start them further away and gradually move them closer to the light. Put your hand under your light, if it is uncomfortable that is obviously too close for your plants. Hope that helped