We add about 2 datafiles of 2Gig each every week with a total of about 300 files.

I find smaller datafiles as opposed to 4Gig file are easier for backup (you can use parallel OS copy using multiple CPU's). You can use multiple channels (up to 15) in veritas netbackup media manager to finish daily backups and refreshes quicker.

Go for 2 Gig files.