G5 boot failed / fans on turbo

[xgipper]

Hi
I had an issue with my G5 Powermac dual 2 ghz machine this morning, which runs 10.5.8, when it failed to boot.
Came into room after turning it on to find loud noise as if fans on some kind of 'turbo' mode, but no boot: screens completely blank and still in standby. This has happened previously a couple of times, but it has re-booted on the next attempt.
This time I repeated trying to boot several times, with the same result. I also plugged in an external FW bootable backup, but it wouldn't boot from that. Nor would it boot from a Tiger disk I had to hand and managed to get into the DVD drive, when I pressed 'C' on bootup.
I then tried the G5 in target disk mode with a G4 machine I have. Bingo: I was able to access the G5's HD and copy across some critical files.
Following this, when I next tried to boot the G5 normally, it worked - and I'm using it now.
So, having just done a complete backup, I'm wondering what the cause of this might be and what I should look into to prevent this happening again?
Any clues?
Thanks

[xgipper]

Well reading around a bit today, it looks like this might be caused by pretty much anything, from a failed battery, logic board or ram to goodness knows what else, so I can see this might be hard to diagnose in a forum. However any help on where on earth to start so I can have confidence in the machine again would be great - or maybe I should just get it checked out by a 'pro', as I'm not very 'under the bonnet' orientated.

[Satcomer]

I had this problem with my RAM going bad. Then after a while I got the dreaded three sleep light flashing before the fans went crazy. This means the RAM is bad and I had to play detective to find the bad RAM (trial & error each RAM stick.

[gsahli]

Why not run Apple Hardware Test that came with the computer?