A clean, well-worn cotton t-shirt, misted with a touch of slightly warm water, and a lot of elbow grease will polish it up nice. Then use canned air to keep it dusted.
BTW, those Sony G400's are quite dirty when they come from the factory. Additionally, it often looks like there is traces of a powdery residue along the edges of the bezels. I suspect this to be the remains of a polishing compound used on the screen surfaces. Whatever, it takes some work to get the Sonys cleaned up. I had one that I swore had a bad spot on the screen, but after several cleanings, I got everything off.