If you are only running one monitor you need TWO identical cards... To run TWO diferent cards you will need to set up another monitor, this option requires a lot of fiddling to get it to work but it can be done...
In order to use multiple graphics cards, there is underlying hardware that is required by both ATI and NVIDIA in order to run their graphic cards solutions. ATI's graphic solution is branded CrossFire while the NVIDIA solution is named SLI. For each of these solutions, a compatible motherboard for the appropriate chipset is needed. Without one of these motherboards, having multiple cards isn't an option.
Once the consumer has a compatible motherboard, there are also restrictions upon what graphics cards can be used together. Originally, NVIDIA required that two identical cards from the same manufacturer, model type and BIOS were needed to function in SLI. They have eased these requirements since then but the same graphics chip model is still required. As for ATI, a special CrossFire master graphics board is required to connect to a second card. This card typically costs more than a non-master board. This card can be used with almost any previous generate ATI graphics board, but performance improvements will vary depending upon the second or slave board"
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