How about
FreeNAS, which gives you an open-ended system, to which you can add bog-standard (P/S)ATA drives as your needs and budget dictate?
You'd be able to plug this (headless, it's managed by internal web page) box directly to your router and NOT get locked into some-or other propietary system! As an added bonus, you can try this out for free to see whether it'll meet your needs before going onto spending actual money!
Quick teaser from the site:
FreeNAS support in the current release:
* Filesystem: UFS, FAT32, EXT2/EXT3, NTFS (limited read-only)
* Protocol: CIFS (samba) , FTP, NFS, SSH, RSYNC and AFP
* Hard drive: ATA/SATA, SCSI, USB and Firewire
* GPT/EFI partitionning for hard drive bigger than 2TB
* Networks cards: All supported by FreeBSD 6 (including wireless card!)
* Boot from USB key
* Hardware RAID cards: All supported by FreeBSD 6
* Software RAID 0, 1 and 5
* Management of the groups and the users (Local User authentication and Microsoft Domain)
HTH
-bdt