Looks like I am not alone with this 8400GS issue:
"Cannot get a PNY 8400GS to work in a Dell Dimension 8250"
"8400gs won't turn on after boot"
The clincher is being unable to enter the BIOS when the 8400GS card is present. On hitting F2, the screen goes blank and the computer freezes (or at least doesn't respond).
Could the card flip to using an invalid vertical frequency when it switches video modes?