When I'm looking for sunglasses, I look for a couple of things:
1) Are the arms thin enough that my headset won't press them into the sides of my head and give me a headache after half an hour?
2) Are the lenses dark enough?
3) Are the lenses big enough to offer protection for peripheral vision?
4) Do the lenses interfere with looking at glass panels (some polarized lenses do, which is why I avoid them)
5) Are the things going to put a huge hole in my wallet?
I'll pick up whatever fits those criteria, normally at my local pharmacy for around $20-30. I could go even cheaper if I wanted to, but the really-low-end stuff feels like crap. If you've got Oakleys already, you should be fine - I wouldn't mess with what works.
I'm kind of breaking rules 3 and 5 at the moment, though, since I'm using a set of clip-on sunglasses that are fitted for the glasses I wear, and thus cost a fair bit more. But it's worth it to not have to deal with contacts.
7 billion, one nation, imagination...it's a beautiful day