Well I can see you have a few options depending on what suits you.
1) Get a SATA Controller card, or new Mainboard that will take more drives. Would need the case big enuf to fit more drives which can be an issue.
2) 1.5 TB drives I see are now on sale (not cheap tho)
3) External cases via USB, Firewire or eSATA
4) A simple RAID setup / NAS kinda deal.
5) Setup a server PC??
I have recently got a Drobo as discussed
here which is now setup and works a treat. No setup of RAID just plug the drives in and away you go. These units support 4 drives, but if you were to use them in the Drobo you would then have space for more in the PC also for future upgrades. This does use some type of RAID but it is all handled automatically so as far as the user can see there is one big drive with the data redundancy.
There are also 8 bay units in NAS/RAID units but would require more setup.
Depends on your budget and requirements as to what is the best for your needs.
Cheers