do you want your light just to see your fish or to grow plants? because many of the hood and light combos you can pick up at petsmart won't cut it for plants (unless you stick strictly to java moss, java fern, and crypts).
if you are considering higher-light plants:
Big Al has a 20"/28w pc fixture for 35 bucks... but then you need to buy bracing legs for it ($7-10) and a glass canopy ($10-20).
another option would be to buy the little 20" incandescent hood that they sell all over the place and put the screw in twirly bulbs in there. (of course making sure they are proper wattage and kelvin rating)
no matter what you decide to do, shop around online a bit first. if you decide to go for a perfecto hood or the incandescent one, petsmart will definitely have both, but will probably overcharge you (even their own website prices are often cheaper than in-store!). so print out the lowest price you find and they'll match it.