I've installed PCIe in Dell 2850 by melting off the back of the PCIe slot, allowing a 16x card to plug into an 8x slot. Works great. I figure I'm just about maxing out the bandwidth with an HD 5770 and going higher would provide diminishing returns. To crossfire a Dell 2850 I'd need single slot cards, which in my case would be a pair of HD 5670. I haven't tried it as it means crippling two other computer.
I have no reason to believe a Dell 6850 wouldn't work with a single card the same way the Dell 2850 does, but it's still PCIe 8x (I think PCIe 1.0) limited and that pretty much maxes me at a single HD 5770 before the bus gets saturated. The difference is in the Dell 6850 I have room to stick a bunch of cards in. Looks to be 3 double sized cards. That doesn't seem to help me much if I can't cross-fire them.
So to answer your question, I am 95% sure I can make one PCIe video card work in the Dell 6850. I haven't figured out if I can use Crossfire to make a second card work for gaming. I have figured out that Windows 7 Pro is limited to two physical CPUs, which is a bummer.