I emailed site5 to ask them whether the 750g accounts could be used for my company offsite backups. The answer was no and it directed me to the site5 Resource Usage Policies on a wiki. It is also interesting to see that there is a limit of 25,000 inodes which equates roughly to 25,000 files. To fill 750g the files would need to be 30 megabytes each.
Conclusion: read the smallprint (and/or the wiki).
Peter's Moblog
If you have a multisite account though, it is 25,000 fiels per sub account.