What files do I need for backing up my wallet? Is %APPDATA%\Roaming\Bitcoin\wallet.dat the only file I need to backup my wallet?

I've heard about backing up every ~50-100 transactions is this correct? I thought that with the wallet.dat file, I have my private key, which is all that I need to restore my bitcoins.
Yes.  The wallet.dat file is all you need to be able to keep your coins backed up.  You'll also obviously need your password if you've encrypted your wallet.

However, backing up the entire folder may be advisable if you're on a slow internet connection.  The blockchain is getting rather sizable and can take a significant amount of time to redownload from scratch.  I recently needed to do that and it took almost 2 full days.