i think i can help...
so temme exactly..wen u plug ur 8400 inside ?
then does ur pc not detect it ?
if it does not...go to bios > advanced and set INIT display first to PCI
if not works....do it to PEG and check out ur ram
i once bought a 8600gt on a asus mobo
installed it...when i set it to PCI..It shows only 512mb ram....1.5gb lost !
and wen i set it to PEG , no video card detected but ram is 2 gb
maybe ur mobo doesnt support some cards...
i bought a cheap gigabyte mobo and the card worked like a CHARM !