The NVIDIAŽ GeForceŽ 6100 is built in and the other graphics card that I bought is a NVIDIA 8400 GS with 512mb of video memory. I have the lcd tv attached to the 8400 GS and the lcd monitor attached to the 6100. The problem is that the lcd tv works fine with the 8400 GS but the lcd monitor is black and it isn't detectable on the computer at all. It doesn't even say anything in the hardware or anywhere for that matter. I know for the fact that the 6100 can be detected if I remove the 8400 GS. I know that the 8400 GS does have a dvi input. Can I use two graphics cards or do I need to get a DVI cable for the 8400 GS?
It is best to run both monitors off the one card. If it has dual output then there shouldnt be a issue. If it only has one and the card will support two you will need to buy a Dongle which is basically a splitter for the monitor cable.
I'd personally be willing to bet that if a graphics card is present in your PCI-E/AGP slot, the on-board graphics card is disabled. You can do multi-head on discrete cards, but only if they'll both operate at the same time - on board graphics cards tend to never work that way.
If your 8400 GS is the card I think it is, you should be able to get a DVI->VGA adaptor on eBay for a couple bucks and run both monitors off the 8400 GS.