First off, nobody NEEDS high-end wheels and they don't make a huge difference (you make the biggest difference). However, if I was 120 lb and looking to build a sweet wheelset for myself, I'd build up a super light set since all manufacturered wheels are overbuilt for you anyway. Unless you're young and still growing, I'd look into getting some low spoke count hubs (16-20 front, 24 rear) and lacing them to some nice and light rims (Niobium 30 for aero, Velocity Aerohead for weight) with light spokes (Aerolites if you can afford them).

As for tires, ride them until they wear out, then get a nice set. Good tires will make a more noticeable difference than wheels, though.