Thanks for the advise and links.
Here's some background on my situation and solution - if your interested.
1) I currently have two 3Ware hardware RAID controllers, an 8-drive 3Ware 9650SE and a 4-drive 3Ware 9650SE in my Server machine. Both arrays are built with 1TB drives in a RAID5 config.
2) The 8 drive array is for my movies (BD and DVD) and it is close to being full (I have about 6TB on it now).
3) the 4 drive array is for music, pictures, and TV shows/series. (I have about 2TB on it)
I want to expand the 8 drive array, but I don't have a place to copy the movies to (and I don't want to re-rip all my movies). So the idea was presented to expand the smaller 4 drive array first, then move my movies to that new RAID, and then I can expand the 8 drive array later.
I just bought four 3TB drives (Hitachi Deskstar 7K3000 series). I was planning to do RAID6 but I found out that I can't do RAID 6 with my 4 drive controller (it requires 5 drives minimum for RAID 6) so I will do the four 3TB drives in a RAID5. This will give me around 9TB of drive space.
Then I will copy the entire contents of the 8-drive array to the new 9TB 4-drive array. First, I will move as much of the stuff from the 4 drive array over to the 8 drive (like my music and TV shows). I might lose a few shows depending on exactly how much space I have remaining on the 8-drive.
When I'm done copying, I will have all my movies moved over the new 4 drive array and all my music/TV on the old 8-drive array. Then I can move the music and TV shows back to the new 9TB array and I will have an empty 8 drive array - which I can then upgrade using eight 3TB or 4TB drives in a RAID6.
That's a lot of moving files around - but this will be the last time I ever expand the arrays. I'll end up with a 9TB array and a second array of 18TB or 24TB (depending on drives used). That should last the rest of my life.
And thanks for the link to the "raid-calculator" too.