In reply to Xharlie:
I have Cat 4 sunglasses (Julbo Nomad) for when it's very hot and bright - I like these ones particularly, because they are a good fit around my face and don't let any light in around them (another advantage of goggles over some sunglasses designs) and the arms are comfortable under my helmet.
I have goggles for when it's colder, windier and overcast, as they're a lighter tint. I also tend to wear these in powder, as there's less chance of a face-full of snow or losing them in a crash. Arguably, they protect your face more, too.
Make sure your goggles are twin-lens at the least, with good anti-fog and venting. Both my pairs happen to be Oakley, but there are many other good brands - it's personal preference on style and fit, but, if you wear a helmet, make sure they are compatible.