Get a Kurobox [aka Linkstation] . People have it running Slimserver and some other stuff. No Jreceiver, I don't know how the Java front is on it. I'm running a Debian chroot image right now, hope to make it a dedicated FLAC ripping machine soon.

• low power - 17W
• small form factor - holds one drive (can attach more via USB2.0)
• budget - $160 for Kurobox, ~$250 for Linkstation with 120GB
• capacity - sorta check, add your own IDE disk to Kurobox
• tech - hey, PPCs are cool
  

Its like a NSLU2, but beefed up. Specs

Heck, be like this guy and get a couple.