>>1
1. You can't
2. You don't want to
3. You don't want an extra 6800 either
4. If you've paid for a dual PCI-E mobo, you've wasted your money
5. If you have a 6800 and think of adding a 7800, no, you don't need an upgrade
6. They day you need an upgrade, a single card will be faster than enough
7. You spend less money on single graphic cards which are modern and fast enough for what you want