If I have
FileVault on, this will encrypt all my data on my HDD. so this said, will this send to dropbox the encrypted files or the original files?
No as Dropbox is running as one of your apps and so sees the unencrypted data.
What you can do is create an encrypted disk image in your dropbox area using Disk Utility (or TrueCrypt etc). Dropbox only sees the encrypted image but you have to mount the disk yourself.
there are other cloud services e.g. SpiderOak that say they store encrypted data and don't have the key to decrypt.
There is BoxCryptor which acts as an encryption layer atop Dropbox and has clients for OS X, Windows, iOS and Android.
Boxcryptor is the right solution for all private and business needs. Once you have installed Boxcryptor, you can easily encrypt files in your Boxcryptor Drive and store them in your cloud provider’s folder (e.g. your Dropbox folder). You will keep control of your data and enjoy 24/7 peace of mind knowing that your data is securely stored in the cloud.