Cook onion, ginger, and salt in oil in a 3-quart heavy saucepan over moderate heat, stirring occasionally, until softened, 3 to 4 minutes. Add curry powder and cook, stirring constantly, 1 minute. Add peaches and tomatoes with their juices. Add sugar and lime juice & simmer, uncovered, stirring occasionally, until mixture is thick and peaches are tender but still hold their shape, about 8 minutes. Transfer to a bowl and cool, uncovered, then chill, covered, at least 8 hours.