Last year hubby bought me a pair of Oakleys to take with us to Hawaii and I LOVED them!!!
I wore them everyday until Tyler got his mits on them and snapped them in two They are a little pricey and run around $100 but they are excellent glasses and offer great protection for your eyes from the sun.
I have a roundish face too and I've been told I look the best in rectangular frames that are black or tortoiseshell in color. You could probably get away with just about any color with an olive skin tone though I am pale as pale can be