Use the OS you are most familiar with. Use any modern version of Windows, Mac or Linux and keep it fully updated.
About backing up your wallet, when storing a backup remotely make sure it is encrypted with a strong enough password to prevent against bruteforce. Assume your adversary can do 1 trillion guesses/sec. The best password is one that is randomly generated and is at least 120bits strong, which is roughly 14-16 characters of letters, numbers and special characters. If you are using a password you made up yourself and didn't randomly generate it will need to be significantly longer than this.
I would not recommend using MEGA, as if you lose your MEGA password there is no way to regain access to your encrypted wallet, so you have to remember two passwords and the chances of you forgetting passwords and losing your Bitcoins is greatly increased. Also as you probably know megaupload was shut down and all data was deleted, so I would stay away from MEGA just in case. Google Drive seems ok.
What would be a good way to store an online backup for my wallet then? Maybe uploading it to Google Drive, or MEGA?
This will only make things worst . Keeping your wallet Online is the worst idea you can get mate .
Hacker don't need to hack your PC , he should simply hack your Gmail or MEGA Account (not even hacking probably simple Brute force cracking) and he is in .
I suggest keeping them in Offline device .
~ Madness
Remote backups are OK as long as they are done properly. They are useful in the event your house burns down or something terrible like that, or if you are travelling a lot.