when should I replace the front struts on an 07 daytona,about 80,000 miles right now and noticing slight vibration at 80 mph

upgrading to better struts is always a good idea- the original shocks put on the car when it is new are not always that good- get some good ones, pay the extra money, replace any worn rubber parts while you're at it (like tie rod ends, strut mounts, etc.) get an alignment and notice how much better your car feels-