Expandable Storage Solutions Media Center PC ?

Always running out of disk space (music files).

Already own 300gb USB/Firewire and already installed 500GB in HP PC
Windows XP Media Center Edition in addition to 300GB that came with
machine. No more expansion slots in PC.

Was wondering about a better approach than USB or Firewire external
drives (slow & expensive). Something I could easily migrate to new
faster PC on next upgrade, like some sort of RAID or disk array where
I can buy cheap 500GB and up hard drives and simply buy a new drive
when I run out of space.

Ideally multiple drives would look like one logical drive, so if I
want to use Windows Explorer to look at folders in alphabetical order,
I don't have to bounce to another drive when one drive can't hold
everything I have A to Z.

Also looking for ideal database application to track music collection
on hard drives and/or CD.

"J. J."