You are going to have an issue migrating this data to whatever solution you choose though. You have 4 1TB drives say full of data now.
To put the drives in a Drobo, QNAP, (any NAS), internal Raid-5 (any raid) you are going to wipe all the data on the drives. You will need to either buy new disks (so you have something to copy the data to), or beg/borrow drives from your friends to temporarily hold the data.
Re Cards - the requirement to run 5 drives is going to punt you into the 8 connector range (your options are typically 1,2,4,8, and 16 connectors). My suggestion would probably be something like this
LSI 9240-8i (comes with cables that turn each of the connectors in the card into 4 Sata ports).
The cheapest option I see on newegg is HighPoint RocketRaid 2680
Edit: Saw all the new discussion while I was typing this. IMO go with one of the various 2TB drives floating around there. The NAS will be slightly more expensive in the end than going internal, but I'm thinking it will be a simpler workflow for you.
As far as migrating the data, i'm not worried. I have some spare HD's laying around plus my roommate has a few TB's in his Drobo he lend to me.
Go with the 2TB drives I linked, they are only $10 more per drive. Also you might not need that PCI network adapter if your mainboard already has gigabit ethernet on it.
My mainboard might have gigabit on it. I'm not sure. It's a DFI Bloodiron P35-R2L I think. I was looking at my board last night and it has RAID functionality but i'm not sure what degree. I may end up getting 4 x 2tb. That would be 6TB of usable space, right?
You don't have a spare PCI-E x1 slot?
$32 - Intel EXPI9301CTBLK 10/100/1000Mbps PRO/1000 GT PCI-E x1 NIC OEM w/ 1 x RJ45
I might. I'll have to check. I have another GPU in the mail (PCI) but last I remembered i'd have 3 PCI and 2 PCI-E x1. I think i'm using up 2 PCI, the other slots should be free once i remove the USB port covers.