PC won't boot, video card?

[jwaves]

While everyone else is having issues with computers starting, I might as well ask why mine isn't either.

When I turn it on, the motherboard powers up, fans whirr, hard drive clicks, cd-rom spins and then it repeats.
Sounds like it keeps trying to boot and not making it to the first step. 
I thought the video card  just wasn't securely in the slot but it's in as far as it can go and there's no sign of life on screen.
I haven't made any changes to the wiring or hardware in months. 

Can a power supply have enough juice to start things up but not boot?

How long do video cards last?
Could summer heat be to blame?

Thanks


Specs:
Motherboard: Gigabyte GA-8I945P
CPU                 Intel P4  1.6GHz
                         2GB RAM
Video Card:     Radeon x550 256MB PCI-Express (4 years old)
PSU:                Turbolink Switching Powerlink LC-A420 ATX 12V

[Crafty]

Yes it could be the psu, but lets try some other things first, is this a new build?

Did you try reseating the ram?

[jwaves]

I built this machine a year ago, worked fine except once or twice, when turning it on I'd have the jostle it around or reseat the video card to get it to boot. 

I tried reseating the ram, nothing going.

Subtle hint:
If I don't have the monitor cable plugged into the card, machine reboots several times in quick succession and then once every half second or so.
If I do have the cable plugged in, the first quick succession does not occur, only the longer half second reboots. 

[Crafty]

Do you have onboard graphics card you could try?

Does it boot in safe mode?

[patio]

Borrow a known good PSU of the same wattage or greater and swap it in there...

[jwaves]

sorry, had to make a tough decision about finding a power supply...
anyway, a comparable working PSU does not allow me to boot

even with the harddrive and CDrom unplugged,
shouldnt I be able to see the BIOS start up if my video card worked?
all I get is black

is there any other explanation besides a bad video card?