Question : Problem: Why does PC boot only to a blackscreen?

High folks.

PC Starts but only to a black screen.  It did exhibit 3 medium beeps so  I think I correctly diagnosed a bad stick of ram and the beeps  stopped after changing it.  But still only a black screen appears on startup.   Another Strange thing is that the PC revs itself up and down randomly like an old car.  From very low RPMS to very high in no particular pattern.
Power Supply is testing with a bad -12V circuit.  2 different testers show it.  PSU is bad.  Changing to A brand new PSU right out of the box gives me the same blackscreen + unstable RPM revving symptoms.  
I have not tried a different video card yet because I am afraid I may do it some testing damage to it, if there is some electrical surging going on.  I am not willing to ruin a good GPU for someone else's PC.  

So do you Experts think this is CPU, Ram, Motherboard?  Or something else?  What could this possibly be ?  

The system is a Gateway 9310S Desktop
MB-----BTX 915G(South Lake 2)800MHZ
RAM---512 samsung (2X256)
PSU---300 watt HIPRO
CPU---P$ [email protected] 800MHZ FSB
GPU---NV GEFORCE 6200 w/ Turbo Cache, VGA+DVI  
Thanks to all in advance,

Donnie616

Answer : Problem: Why does PC boot only to a blackscreen?

trouble is the cost - to check the CPU you need another working motherboard and vice versa.
plus your time added to the cost.
as its a afriend - you can  probably go the extra mile - but the fan speeding up and down and the lack of regular booting noises indicates either or both MB and CPU are 'brown bread' (dead) as we say in cockney rhyme.   you have to tell your friend in your opinion what you believe and suggest if they take somewhere else to get a quote.
Once I have tried a new power supply some spare ram then I suggest that the hard disc is housed in an extra enclosure and they buy a new PC and I transfer across the data.
if you take in to account the hours spent debugging the problem and the potential problems if it fails 2 weeks later  or if you damage any new items you try.

one final thing...
for the motherboard - look at the capacitors (round cylinders) on the motherboard are the flat topped or bulging - see http://www.thenakedpc.com/dan/Bulging_Capacitors/index.html