Honestly, if you're looking to upgrade your lighting I would go with a 150w mh over the T-5's. You would have to go with 18w T5's to have them properly fit over your 20" wide tank. Not only would the wattage be quite a bit lower, but I'm pretty sure there isn't much of a selection of 18w T5 lamps yet.

If you did go ahead and go with the 24w'ers I would think about doing a 6 bulb combo. The tank is fairly deep top to bottom and I'm not sure that the 4 bulb combo would be enough light even if you replaced the stock bulbs from either fixture (which you would absolutely HAVE to do for your tank). 6 bulb fixtures can be pretty pricey. Once you start to get into that range you might as well go halide.

get yourself a 150w sunpod, stick a phoenix 14k in that sucker, and call it a day!!!

my 2 pennies.