Honestly, I wouldn't get either. I don't know a lot about Xeons, but a double CPU slution will rarely be better than a high-end dual core single CPU.
As for the 6800, you'd be wasting your money. You mention overclcocking. Now, you can save yourself like $200+ by buying the 6700, and just overclock it to 6800 speeds. The 6800 won't overclock that much further than the 6700, no matter what anybody tells you about unlocked multipliers.
The AMD X2 4800 and the FX60 is a good example of this.
Get the 6700, and spend the money you save by buying a second GFX card to go SLI/Crossfire. You'll see a bigger difference in games with SLI than with the 200MHz difference in your CPU.
Just my opinion though, feel free to dissagree........