Windows Server 2008 Backup planning

With Microsoft's decision to discontinue NTBackup and force us to use the new Windows Server 2008 backup solution (without paying for third-party stuff), I am thrust into a position to rethink our planning for deploying new servers to various Small Businesses.  Now that NAS' are not very useful (only repetitive Full Backups via command line), I was wondering if anyone had a good idea for best practices.  I will be creating servers for small businesses, but would like to know what size drive I should get for the backup.  Theoretically, if the user has a 250 GB hard drive (but uses only 30% of it), will another 250GB hard drive be enough to carry a full backup with fairly good incremental retainment?  I'm a little shady on how the new Server 2008 Backup system backs up all the files, as the main purpose of having this attached backup is to be able to restore to early files in case of end user error, which daily Full Backups only will not accomplish.  Thoughts or advice?  Should I get a drive bigger than the capacity of the drive I'm backing up?