Monitor not turning on

Hello,
My monitor this morning was not responding to my computer this morning(worked fine last night). Getting power, but monitor doesn't pick up any signal. I tried the monitor on another computer and it works, ruled that out. Next I though maybe video cards(2 x nvidia 8600 gts SLI-ready), but the only other computer in the house is a pre built comp from HP bought from best buy.
Experiment with other comp:
It has an onboard video cardIt had 1 PCI-E port(one of my vid cards fit so I'm assuming its a PCI-E port), but the PSU doesn't have a power connector for PCI-E, but the vid card seems to turn on even without it(mobo power runs it?) cause the fans turn on. Turned on the comp and nothing, no signal. Does the video card need to have a direct power supply from the PSU in order to run properly, or is just running off the mobo enough for it? I made a guess that since the PSU didn't have a PCI-E cable for the vid card, it wasn't working properly. I tried it for both video card, and no signal. Just bought these like 2 years ago!
Another assumption:
I've read that the motherboard could also be an issue with the monitor not having an signal. I have an ASUS M2N-SLI wireless edition motherboard. I keep hearing about beep codes, and haven't paid attention to my computer beeping until now(of course). I've read that the M2N-SLI does have beep codes. So what I did was remove the RAM and vid cards(CPU stayed in), and turned it on and it just shut off after like 2 secs, and I guess it sort of beeped... but could be just everything shutting off instantly. The light on the motherboard is still lit, and I replaced the battery, but still no signal.
My CPU: AMD Athlon 64x2 3200+ Socket AM2
My motherboard and CPU are old, and I can understand them breaking down after like 5-6 years. What could be the problem? Video cards or motherboard? Please let me know if you need some more description on certain areas.