You have 3.5 sq. ft. there:

Length x Width = Area

24in. x 21in. = 504 square inches

504 / 144 (12in. x 12in. which is a square foot) = 3.5 sq. ft.


... I had a 400w hps in the same area (that's about 55,000 lumens total) that worked out really well in terms of lighting, so don't believe your buddy.

T5's are only effective about 1-3 inches away from the plant, so mounting them on the sides as primary light sources won't work out too well. If it were me, I'd get a 250w hps, maybe a cooltube for it, and a good inline fan to cool the cab. CFL's will work also, but they also need to be kept close to the plant to be effective.

Check out some of the growlogs like this one: http://boards.cannabis.com/grow-log/...ard-cab-9.html, might give you some ideas.