Tom Tancredi :
Have you looked on www.PCpartspicker.com Always want at least a BRONZE certified PSU, never EVER go CHEAP on a PSU, it will cost you more in your parts being damaged then to properly spend the money the first time.
As for the comments on the 560, I call BS on it, per the manufacturer
http://www.geforce.com/hardware/desktop-gpus/geforce-gtx-560/specifications .
Now if your using a cheap / low quality psu, that 500-600w may actually only outputting 350-450W and depending on other hardware (multiple HDDs for example) wouldn't be enough to power it.
If the 560 isn't working and your Mobo is saying something wrong with the card, then something is wrong with the card. Before looking at 'other parts' as the problem, simply try with another video card to see if it works. Do know HARDWARE only warrantied for 3 years and that is over a 5 year old card, it could have just died too.
Craig M :
I figured it may not be detecting the graphics card at all now due to lack of power getting to the card from the power supply.
Not possible. Again NOT your PSU, totally is the GPU that has a problem.
Craig M :
Could I unplug the hard drive (only have one connected) and see if the computer will turn on? Or would that have very little impact on it?
To get to BIOS, sure, but again has nothing to do with the GPU being faulty.
Craig M :
I'm going to have a guy that works on computers take and test the card when I can because I don't have another computer to test it on. If the card is deemed good on another pc, should I then look to replace my PSU? thanks
NO that doesn't mean that specifically. Your MOBO could be dying / failing just as much as the SLOT the GPU fits in could be damaged (can't count the number of people saying the SAME THING, and a look at the GPU slot showed the 'pins' were bent / rubbed away / worn out requiring a new Mobo).
Again the BEST test is to TRY another GPU in YOUR PC. See what happenes. If that GPU functions but not this card, then either it is Power issues OR the 560 doesn't work. Then you would try the 560 in a new PC, does it work there, if SO then yes Power, if NOT then 560 is dead.