i keep sps under 2X24 watts...but watts is not a good estimate. Par and lumens are better but PAR is kinda hard to measure unless you already own a PAR meter or you are willing to buy one. The 3-4 watts guide never helped me...lps is supposingly easier than sps, probably because they are hardier...but sps isnt that hard.
HAHA...that fixture is really worth the money, would have gotten it instead of an aquazonic fixture...the cooling fans and the individual reflectors are a real plus. T5 is still one of the cheapest alternative around, unless your thinking of metal halide or LED...lol If u plan to use T5, the bulbs u use really make a dif...the qian hu one sucks...i prefer the arcadia one.