NAS & Additional Drive for RAID 5?

Hi,

I purchased a Go Flex Home a while back with the intent for it to be a storage device to create a media server out of -- proved to be woefully inadequate (half the time, my PS3 doesn't see all the media on the drive, and half the time the file types don't play on the PS3 either).

I've decided to bit the bullet and purchase a decent NAS and a media player to get the job done. I'm looking at either the Synology DS411 or the QNAP TS419P-II along with a NetGear NeoTV 550 Media Player...

My question is when it comes to drives in the NAS -- given the cost of hard drives right now, I'm looking at buying three 3TB drives and putting them in RAID 5 for 6TB of space. What I'd like to know is if the time comes that I'd like to add a fourth 3TB hard drive, will the added drive screw up the RAID? I know the parity is distributed across the disks, but the added disk may complicate matters, and I'm just wondering if I'll have to re-create the RAID if I add a drive in the future.

Also, if anyone has any suggestions when it comes to NASes that may save me some money, I won't complain either

Thanks in advance.

Both the Synology and QNAP allow for RAID expansion without loss of data.