Sorry for reading your message so quickly. I think I understand the situation better. I operate an 840GB Raid5 with a networked off site backup and have happed on this once before.
What I would recommend before doing anything is buy two 500GB hds ($100 each) and a usb to IDE (or SATA) and making a static backup of the data before doing ANYTHING. You might be able to get away with the mirrored server but if this resizing takes any significant amount of time or trickery you run the risk of that raid going down with no redundancy. So for the money I would make a quick static backup.
In terms of expanding your raid I'm assuming you had a hardware Raid5 card with 1 logical drive (at 1 TB) and your replacing your drives with a larger set to increase your overall size. What I would do is keep the file system the same and add another partition. If the type of data your storing allows it. My raid has two primary partitions right now. I'm fortunate in that I store a lot of small files in many sub folders so when one directory got too big to fit all on one partition I moved one large sub-folder to the other partition and made a symbolic link back to the other drive. This approach might work for you as well depending on the type and size of the data files your storing.
The only time I've tried to expand a file system was using the parted libraries and that had a seg-fault at 700GB. (thank god for Backups
) so I would avoid the problem all together if possible.