Awesome build! I really like how your tubing is so organic feeling, it's usually just a bunch of right angles. You should consider a big magnetic filter for the bottom so you don't have to take the thing apart to clean as often.
I had the issue when I switched motherboards a while back, it worked fine with my old one but the new one it didn't. I switched to the HDD indicator LED pin on the front panel header and now it only lights up with the system and doesn't even flash, which I was thinking it would before I tried.
It loads for me. Very cool mini workstation build with heat pipe risers connecting the xeon and quadro to 2 factory heatsinks via heatpipe risers.
Edit: Made an imgur album for those who can't veiw pictures http://imgur.com/a/nu9TY
This is the 2003-2007 Apple Keyboard (model A1048) that would have shipped with late MDD G4s up through early Mac Pros, as well as whatever iMacs they were selling in that era like G4- early intel models.
The keyboard that this G4 shipped with would have been an Apple Pro Keyboard with black keys.
Silverstone Argon Series AR05
Specs: 104mm (W) x 92mm (D) x 36.7mm (H) (with fan)
Fan: 92mm x 92mm x 15mm Riffle Bearing Fan 20 ~ 28.3dBA
Silverstone Argon Series AR06