2) The graphics card is the real engine for good gaming. To that end, a stronger graphics card than the 6850 would be a more appropriate balance.
With your budget, though, and looking to future upgrades, the 6850 is not bad.
3) Cases are a personal thing, and a cheap case is functionally not a bad place to economize. The one you picked does not have good enough ventilation for a gamer.
80mm fans are ineffective and loud. Side fans are not the best for good airflow. Add to that, the cost of buying such fans, and I think you can do better.
One perennial favorite is the Antec 300 illusion model. http://www.newegg.com/Product/Product.aspx?Item=N82E168...
4) The corsair 600CX is a good one, and will power a 7970 or GTX580. But, it will not power any graphics cards you might want to run cf/sli with.
5) Permit me a perhaps unpopular rant on
Dual graphics cards vs. a good single card.
a) How good do you really need to be?
A single GTX560 or 6870 can give you great performance at 1920 x 1200 in most games.
A single GTX560ti or 6950 will give you excellent performance at 1920 x 1200 in most games.
Even 2560 x 1600 will be good with lowered detail.
A single 7970 is about as good as it gets.
Only if you are looking at triple monitor gaming, then sli/cf will be needed.
Even that is now changing with triple monitor support on top end cards.
b) The costs for a single card are lower.
You require a less expensive motherboard; no need for sli/cf or multiple pci-e slots.
Even a ITX motherboard will do.
Your psu costs are less.
A GTX560ti needs a 450w psu, even a GTX580 only needs a 600w psu.
When you add another card to the mix, plan on adding 150-200w to your psu requirements.
A single more modern 28nm card like a 7970 needs only 500W.
Case cooling becomes more of an issue with dual cards.
That means a more expensive case with more and stronger fans.
You will also look at more noise.
d) dual card support is dependent on the driver. Not all games can benefit from dual cards.
e) cf/sli up front reduces your option to get another card for an upgrade. Not that I suggest you plan for that.
It will often be the case that replacing your current card with a newer gen card will offer a better upgrade path.