affa is the answer (be aware it can have some bugs)
or you can think about a DIY solution "affa style" using rsync and SME's events: setup a second server, install the same contribs but don't configure them.. rsync the files/dirs neede (the same that are in standard backup, see wiki for info)
if your main server goes down, you can "raise" your backup one in minutes (if you know how SME works internally)
you have to know which files/dirs are involved and all the involved events (pre-backup on the source server, pre-restore and post-upgrade on the destination one)