Onboard LAN Card broken and OS won't boot after installing a new/different card

I have a Dell Optiplex 755 running Win XP with what I suspect to be a broken integrated Gigabit LAN card.  All of a sudden the network card just stopped working.  It won't recognize a link accept thru an old 10mb hub, but even when it has a link it won't obtain an IP address and I can't get any traffic to pass by manually assigning an IP.  I'm almost positive the network is broken, but to verify this I'm trying to install a PCI network card.  

I've tried two different cards and neither have worked.  As soon as I install the card, the computer won't boot - not into safe mode, not even into the bios.  It just entirely locks up after pushing the power button and displays nothing on the monitor and some of the lights on the front of the machine stay lit.  As soon as I remove the card the computer boots just fine every time.  I've verified this with two different cards.  I get no beep codes.  

I've checked the bios as far as enabling the PCI slot.  I've even tried to disable the onboard LAN.  But still, as soon as the card goes in nothing works.  I've tried system restore. I've tried Dell Diagnostics.  Dell support won't help me unless I get service in Latin America b/c that is where the computer was purchased - it isn't under warranty anyway.

All I want to do is install a second LAN card - doesn't seem like it would be that hard.