A Cessna 152 like the one in yesterday's incident has a useful load (fuel, passengers, and any other disposable weight) of about 500 pounds (give or take 30 pounds or so) depending on the installed equipment. If you stripped all unnecessary weight, used a light pilot and minimal fuel load, you might be able to carry 350 pounds or so of explosives. Of course, the fuel itself makes a pretty good explosive and you might get away with taking off over the normal gross weight. Any way you cut it, you'd be hard pressed to carry over 400 pounds of explosives in a 152. If that was a dirty bomb or some form of chemical or biological weapon, you could do a lot of damage. Otherwise, the plane would do very little damage to the Capitol building. Given it's low speed (perhaps 120-130 knots in a power dive), low lifting capacity and light weight, a 152 would be a poor choice as a terrorist weapon. A twin like a Cessna 310 or Piper Aztec would be a much better choice.