i used to use the window sill and the results were lousy, spindly plants, few leaves. i made a grow light stand, 2 shop lights side by side, 2 shelves (could use a 3rd at floor level but never needed to) and the results were amazing. i raise almost everything i grow under these lights and even pole beans this year vs direct seeding for a jump start on the season.
i've posted this before but here it is again.
http://www.tsflowers.com/plantstand.html
tom