Former Washington Redskins linebacker LaVar Arrington was in good spirits and stable condition yesterday after crashing his motorcycle on Route 50 Monday, but remained under observation at Prince George's Hospital Center, according to his agent, Carl Poston. Poston, who visited Arrington in the hospital last night, said the former Pro Bowler would be held for 72 hours from the time of his arrival Monday morning to ensure there were no infections and that his vital signs remain strong. Poston said that Arrington, taken with the second overall pick in 2000, fractured his forearm but did not break his leg, as Poston believed was the case Monday. Arrington's leg was in a cast, however, because of a severe laceration. He was not being treated for head trauma, Poston said