Encrypting sensitive data on Mac + Windows
Consider using Wuala instead of Dropbox. It's the same service as Dropbox but has integrated encryption. Then you don't have to worry about encrypting before upload and decrypting after download anymore.

Disadvantages of Wuala are (in my opinion): a worse user experience, and user interface (which may be due to the fact that there are less interface designers and more encryption engineers at Wuala); and a higher memory usage (which is due to the fact that Wuala is Java based, and the encryption algorithms come at a price).