I have several ASD's, specifically one that is entitled "For G5 Systems only." The Mac will only recognize and boot from the CD when I press and hold the power button until I hear the long tone (Open Firmware) but then freezes or will give a message saying that the CD cannot be used on this system. Also, the RAM is known good, but I know these machines are picky. This machine is so not representative of Apple products.

There's two final versions of the ASD for G5 systems. Neither will boot the wrong G5.
2.5.8 will work on the most models.
2.6.3 will only work on the newest models. (those with PCI-Express slots) I suspect you have that one (version 2.6.3). You need ASD version 2.5.8 instead.
If you have replaced the logic board, without having a good version of the ASD to use, then that can explain at least one of your problems. The ASD has the processor calibration utility. If you don't run that after replacing the logic board, the fans (all of them) will run at full speed continually, and there's no way to get those fans under control without running the processor calibration.

Resolved Powermac G5 won't boot - no chime
 

After searching the internet for days I did finally try what I thought was an unlikely cause to this problem -- I changed out the video card. This solved the problem.

My specific problem was that when I pressed the power button the light was lit (no red diagnostic lights on the logic board) but after a brief spinning sound there was no startup chime and there was silence until the fans started up full blast. I had been using the computer and it suddenly turned itself off and I was never able to restart it until I replaced the video card.

If you want to test to see if this might be your problem -- before you spend money on a video card -- take the video card out of your machine and press the startup button. If you get the normal powering up sounds with normal fan noise it is likely the video card.

I had read this solution in one or more posts to this issue, but I couldn't see how that would work so passed over trying it in favor of other possible causes. I am posting this so others will take note that this is good advice even if it seems unlikely.

Good luck!