I'd go with a 250w. With only 2 feet of vertical growth room, a 400w wouldn't work because you need the canopy to be a certain distance away depending on the temperature in your area. Too close and the plants burn. What USDA hardiness zone do you live in? If you don't know, you can google it. If you're in zone 7 or hotter, I'd just go with the 250w. Since you already have a 125 though, it will work. If you want, you can even add in a couple of CFLs.