These work for me:

http://www.macgurus.com/productpages/sata/satakits.php

2 drive SATA enclosure = $136, 4 drive = $254, 8 drive = $451.  (They also offer 3-, 4-, and 5-drive enclosures.)

Get the drives & SATA card from Newegg.  Instructions for slapping it together:

http://www.macgurus.com/productpages/sata/sataguide_1.php

The great thing about SATA is that you can do anything with the drives in Windows: spanned, mirrored, striped, RAID-5, JBOD.  Some SATA cards have HW support for RAID (e.g. 0+1).  If a drive starts to flake out, you'll get error messages in the System Event Log (unlike a NAS device, where you might have to set up SNMP).  Also you get all that SATA performance (not limited to NIC speed, doesn't add network traffic, don't need another IP, etc.)

You can get 6' SATA cables so the array doesn't have to be immediately adjacent to the computer, and they are a LOT cheaper than SCSI cables...

When you want to increase the volume size, just throw another drive in the enclosure (or buy a new enclosure) and extend your NTFS dynamic spanned volume.  No need to reboot.

If your computer blows up, just plug the enclosure into another computer and import the foreign volume.

MacGurus also has removable enclosures.

Disclaimer: I don't profit directly or indirectly from the suggestions made.  Just a satisfied customer. Big Smile