There are a lot of good questions in the "other" section. All of this boils down to: Depends on what you're doing and how fast you need to recover. Instantly? Very expensive. Can you be down a week? Probably pretty cheap. How often do you get physically next to the server?
But here's what you're probably going to want to back up, at a minimum:
any other application files
any database files
For most of this, you can either use rsync or tar. Rsync will require a server "live" somewhere that you can host the data on. Tar will make an archive file (Which you can bzip) and then you'll have to extract it on another server.
You'll definitely need something with enough disk space and cpu and whatnot to run your failover server. It'll need to have RHEL5.3 (keep the same patches on it). You probably won't have to license the 2nd server unless you're running it all the time.
For the database files, you'll probably want to start with mysqldump or pg_dump. If you have Oracle, start with rman. These are the most inefficient, but the fastest & foolproof-ish way to get backups of these data.
Hope this helps as a place to start.