bfg nvidia 5500 troubles

hello,

i need some help with a videocard i am trying to install.the card is a bfg nvidia 5500 oc. Motherboard is asus p5s800.
The board has onboard graphics.when i put my card in it post fine uptill windows boot screen the screen gets very dim and just hangs there
if i go back to the onboard it works fine. i have went through bios to disable onboard video card and tried reinstalling it with no luck.
it seems that the pc does not even see it.
i am pretty sure that the card is good as it worked in anouther machine. any advice is thanked.

I have found this problem a few times with onboard graphics.A good trick that sometimes works is let it install the onboard but disable it in windows.Hope that works!

Thank you glynn but it did not work

my pc specs are
pentium 4 3.0 775
asus mb
512 mb of ram
480 power supply (came with case so not sure of brand)
120 gb western digital hard drive

Did you set to use AGP in your bios when you disabled onboard graphics?

just wanted to say thanks
went through and rechecked everything.i didnot set bio correctley.
thanks for pointing it out