The following table represents the Windows 7 editions and each edition's major features.  As you can see, the only real choices are the professional and ultimate editions.

Data Sensitivity - Ultimate Edition

If the data you maintain is super sensitive, consider the ultimate edition which encrypts the entire drive and file contents dynamically; even if the machine is stolen or a virus gets access to your files, the receiver of this information wouldn't be able to use it.  Of course this complicates the recovery or undelete of directories and files that you may need to recover; any recovery utility would have to support the unecryption, retrieval, storage and re-encryption of the recovered info.