Hi!
If you only want to use it for backup place, then why would you buy X GB ram, etc. You only need a minimal ram to run the operation system and handle the writing of data to disk, thats all, all the gzipping and cpu intensive script should run on the machine where you are backing up from (at night for example, when you don't have that much visitors)
You create the backup, ftp/sftp etc. it to the backup host, and you are finished. This way you can have backup host just for a few bucks.
Cheap backup hosters i know:
hostigation
buyvm