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Keeping copies of you physical XOOPS files shouldn't be too hard a job assuming you aren't adding/modifying/removing modules, themes or hacks. However, keeping the databases syncronised on a budget without loosing data is a little tricky. you also will have the problem of DNS propogation. Most standard DNS records have a TTL (Time To Live) of a couple of days, so if your site goes down and you are wanting to use your backup site it may well take upto 48 hours for the changes to fully propogate around the world. Even if you set your DNS to use a very short TTL it can still take an hour or two to take effect everywhere. You can solve this if you have advanced setup access to configuring you DNS though. It really depends on who is hosting your DNS.
As for syncronising you database, you could use an automatic MySQL backup manager to keep things tight, but then what happens if a transfer bombs out half way through? BD load balancing is really not my area though, so hopefully someone smart can help us