With the changeover from analog to digital TV signals less than a month away, many of us are looking at TV upgrades to match. Given the choice between an LCD or a plasma, however, it's hard to know which to pick. Both use more energy than older CRTs (cathode ray tubes, the onetime standard for television technology), and both are made with nitrogen trifluoride, a chemical that lingers in the air for up to 550 years and one some climate scientists say has a bigger impact on global warming than the world's largest coal-fired power plants. The most earth-friendly decision you can make is to stick with your old CRT until it bites the dust, but when it's time for a new one, is there a truly greener flat-screen TV?