ASUS P5KC Mobo startup problems

Hi everyone, I'm having a very troublesome issue with this motherboard (or Graphics Card) - I bought a Nvidia engtx275 GC at the end of last year but whenever I insert the card my pc doesn't want to boot up. There's a green light from the mobo but no familiar beep, also the display looks corrupted if it does boot up. I've taken it to some "pro's" here and they said that it could be a underpowered power supply - I've upgraded to a 850w cooler master and still i have a problem. I've swopped graphics cards (tried out a Nvidia 7900 as well) and the same thing happens, I've currently got the pc running on a nvidia 7300gt graphics card. Can anyone relate to this problem? Has anyone ever experienced anything like this before?
Here's a full list of my system if you need that:

E8400 Dual Core CPU
4 gb of ddr2 800 mhz ram
Asus P5kc Mobo
Nvidia 7300gt graphics card (currently installed but really want to use my 275 GC)

Any advice would be greatly appreciated.

Thanks.

Hello and welcome to the forums
Does your PC work completely fine with the 7300GT installed ?
Also do the 7900 and GTX 275 have the same problem?
Try a clean install of your VGA drivers,here is how(have your GTX 275 on the motherboard),here is how
1_Download the latest VGA drivers,then Download DRIVER SWEEPER(from guru3d.com)
2_Go to control panel and uninstall the driver(older one) from there
3_Reboot and go to safemode and open driver sweeper and click on clean
4_Boot normally and Install the new driver

Is it possible for you to test the cards in another system ?

That's a good idea,let us know if it works fine or not