Halve the peaches, remove the pits and stems, and dice them into small cubes. Finely chop the red onion, and mix with peaches in a medium size bowl. Chop the cilantro as well, and add that to the peaches and onion. Juice the lime, and drizzle over the top of all the ingredients. Sprinkle the sugar into the bowl, and toss the salsa. Refrigerate for about 30 minutes before serving.