important files backed up to "dropbox" which has six computers linked across four sites (home, work, mums, friends).
some of which only attached/syncd once a week.
If its important, worth considering another solution (in addition), as you still have a single point of failure - Dropbox. The connected machines will sync with what is on Dropbox, so a cock-up their end (I know of one major UK based cloud storage solution losing a load of customer data) or user error your side, will impact all files. I know you say some are only attached at certain times, but thats not foolproof.
A 2nd cloud storage account syncing the same local folder is not the answer either, for the same reason. Thats the downside of syncing, as opposed to copying

Obviously, it depends on the definition of "important files" as to the lengths you want to go to. But cloud storage (via automatic syncing) isn't really a viable safe backup solution. There are cloud based backup solutions that can work, using processes not dissimilar to traditional backup methods, but these tend to be quite expensive for usable amounts of capacity.