You did not mention the average daily increase – industry average would be about 1 percent per day for very rapid growth = 250 to 350 percent each year (do you work weekends). Or medium growth of 1 percent a week, I hope this is your model or you are living on borrowed time. NAS or SAN is the best solution when combined with LTO or DLT tape libraries. Full time backup online, Daily incremental backup to tape, and weekly full backup to tape with at least two copies (one onsite and one in a bicycle set to the main office). Hopefully you also have disaster recovery plans which include backups of main and branch office data stored at a third secure site. After 9/11 the standard best practice for insurance has been to have the data that is your business secure and saved offsite. May all your problems be musch smaller.
At the point you have to double your drives at the branch offices you will have to have high speed connections. Then look at intertie between the NAS setups.