I have a PowerEdge 1950 with 2 x E5405 2.00Ghz CPUs and 4GB of RAM sitting around. It's currently not doing anything at all so it's a perfect fit for this task.
I have traditionally used 7-zip's command line tool on Windows to encrypt files that are no more than 5 GB. On my PC this doesn't take much time so it hasn't been a problem. Now, however, I need to start encrypting backup files that are 500GB+ in size and this will take a significant amount of time and resources if done locally.
I do not have a time limit for the encryption process so I could simply start 7-zip and let it run for a day on my PowerEdge but I'd really rather find a more efficient OS+tool to do the job.
On Windows I've tried AxCrypt and Kryptel but they were not any faster.
To clarify this question a little... the heart of what I'm trying to get at is suggestions for proven tools and OS (if that matters) for encrypting files that will be sent offsite for archival storage. In my case I will be sending them to Amazon Glacier so full-disk encryption is not appropriate.