Can i install a Gainward 512MB 6800GS graphics card into a P4P800 Motherboard and will the BIOS detect the 512MB of RAM on this Graphics Card or will it be just 256MB. Can i upgrade the BIOS to solve this problem?
Yes, you can install it... I have the same mobo and I was looking at that same card to buy for my system, but I went with the Gecube x1950xt w/ water cooling instead... It's way faster... I think that there is a option in the BIOS (in mine there is) to set the arpature size... So yes...
You can put it in that motherboard (although I'd recommend against watercooling it unless the rest of your system is) but check whether your Power supply is up to the job first.