At present the USADA charges mean very little in official cycling terms until his Tour de France wins are removed by the UCI.
In terms of legacy I believe Armstrong will be known as the Cancer Survivor that won the Tour De France which will be an inspiration to so many people- whether they suffer than cruel fate of cancer or use him as an inspiration for later in life.
His cycling legacy is that he has bought cycling to the mass public in North America. 15 years ago a doping case like this would have been merely a small column near the back of a newspaper in the USA. Now it's front page news. He's significantly raised the profile of the sport throughout the world and cought so many people into the sport that his legacy will live on throughout his lifetime.
Whether or not you agree with the charges bought against him and their nature or legality it's hard to argue that any one person has done more to promote the sport of cycling in North America