To answer your question, I would suggest a HD6950. You can unlock this so it runs at 6970 speed.
If you prefer NVIDIA cards, you need to run cards in SLI. ie- two cards.
If you go AMD, you only need one card. However, depending on your monitor resolution, usage (ex gaming), you may need to run crossfire (ie 2 cards) to get adequate performance. If running 6 monitors, you will definitely need a second card if gaming.
You will also need an ACTIVE displayport adapter to use AMD cards in Eyefinity.