Jungle Disk looks quite good although there are some drawbacks I see in here http://www.jungledisk.com/faq.shtml Nothing too serious if it's truly just an archive, though. If it's an archive where you're really just going to upload once and forget it except on the off chance you need to get something out of there, this should be perfect.

The main drawback of online storage in general is that you can only upload to it as fast as your connection's upload speed, which on many types of connections is much slower than your download speed. So 50GB is gonna take some serious time to get uploaded.

Other than that, though, looks like it's even cheaper than an external drive, at least until several years go by....