So about the abs?

Ok I was just wondering about the abs. Im 6'2 and 120 pounds so fat isnt a problem. Sadly Im doing this to try to gain some wait since muscle does weigh more then fat. Just to clarify I dont have an eating disorder or anything. I eat all the time. I eat the worst junkfood ever too and usually set on my yet no weight gain. Though my dad and grandfather are the same way very scrawny and lanky I guess you could say. I perfer "wiry". Anyways for the last couple of weeks Ive been doing about 100 crunches/situps every night. This week I wasnt getting very sore so I upped it to 200. So Im wondering in the end how many will I need to be doing to mantain (and get) a decent six pack? At my current rate about how long will it take to see some real definition? Already I do have a very slight raise in my uppermost ab that wasnt there before. Yes, I know its an ab cause when I poke it screams with soreness. Also how many times a week shout I do my routine? Im just wondering if I can over do it.