Ok. Did you have a card in there before and if so, what was it?
If you had a card in there before, was it an ATI card or NVidia?
If it was an ATI card, did you uninstall the ATI drivers and install the NVidia drivers?
If you didn't have a card in there before, did you disable the on-board graphics in BIOS?
What motherboard do you have?
I'm not discounting the power supply just yet, but there are a lot of other questions that need answering.
right well i figred as much... lets see
I had (and have) another card in there currently, Nvidia 7800 GT
Mother board is an Azus Fatal1ty mobo... im sure exactly how to check that but i believe that thats it.
Having the 7800GT in there and working pretty much blows all my thoughts out of the water. Sorry. That Antec 450 does have it's own PCI-E power cable, doesn't it? Last gasp would be to contact your friend that gave you the 8800GT and ask what power supply he has and if it would be possible to test your system with his power supply.
Edit: Oh. And I'm assuming you didn't switch monitor connection ports and that you're using the same DVI (or VGA) monitor port/cable as you did with the 7800GT.
i dont know if this helps at all... but every time I try to start with the 8800 in my bios and clock resets... i ahve no idea why that happens... and yes its got a cord I and haven't changed DVI cables
nono.. i cant get to bios with the 8800 in... after i pulled it out and put my old card back in i gota boot, and the Bios was flashed.. like it didnt have enough power or something... gona get a new PW suply... my old one isnt that great anyways.
I'd say it's the power supply. Sounds like it's either too unstable or just not enough juice, I'm leaning toward unstable since not having enough juice shouldn't reset the BIOS, unless the battery on your motherboard is dead too.
well it isnt the power supply... i just bought a new one and tried booting (system does spin up tho) with the card in... got the same thing as last time... no boot, and no signal, and a checksum error when i put the old card back it... so the card is likely dead, sigh there goes the money for the power supply