navision 3.70 database expand -how?

Hi, I have navision 3.70 with its own database running on a windows 2003 server, We have 16 disks 8 GB each and on every disk is a 4GB *.fdb File. My whole database has 64GB. We want to expand it to 128 GB. Question: Should we for some month only expand with 32 GB (16x 2GB) that means 6GB *.fdb Files and later when we need further space expand each File to 8GB. or should we at now expand each *.fdb File to 8GB? would it be for the performance better when we create empty new 8GB *.fbk Files and the restore with the client a navision Backup. Thanks