Nice if you are thinking about the future. Say your buy a PCIe 6800GT for $400. That's the 3rd best card on the planet right now. Well 10 months later the new release is out and your card is looking pretty old. Well slap down maybe $150 for a new 6800GT and you are most likely faster or just as the fast as the new refreshes.
But as of now there is no point in owning 2 6800s.