If you run an idler then you really need to use a "toothed" V belt.
You can't go wrong with a March kit. For $840 you'll have a beautiful set up with all new pulleys, everything matching perfectly and a single serpentine belt. It puts a large "wrap" on the alternator, so even if you upgrade to a 140 amp it will never squeal. It mounts the AC compressor low, on the passenger side, and out of the way so you don't have big ugly hoses running over your pretty engine.
It's one of my favorite parts of my build.