For regular desktop use the single 6850 will run 3 monitors reasonably. However for gaming if you really intend to play at 3x1680x1050, you need to be looking at least a 2nd 6850 for Crossfire, and even then it probably won't be enough to run any high settings. Generally a rig that is intended to run such a setup would be recommended to have two 6950's with 2GB memory which by themselves cost about 250 each and then a 750W PSU would be in the right ballpark to run a suitable setup with those cards.
As for the rest, currently Z68 based mobo with SLI/XFire support with i5 2500k and 8GB of ram is the way to go on that front, but even the 900 will be a stretch if you put in the 2 6950's. But yes, just giving you some generic info, maybe someone will give you a whole setup list.