Help with old computer

My son has a storebuilt computer witha 600 Duron, a K7R board, a DVD rom, an old cdr burner. 250 watt power supply. When trying to add a new hard drive to supp his old one, the computer cut off immediately. We tried to put in the new drive alone and the same thing happened. We also tried a few other drives and the same thing happened. When we replaced the old drive alone it started up. We think it is in need of a higher wattage power supply. Do you thing we are correct? (We checked the cables and the set up all looks good).