If you have the budget and are interested in overclocking (there are plenty of guides) get the 2500k and a pg7 board.
Get a x-fire certified board too. The 6870 is a decent card and a second one in the future would make a nice addition to your GPU performance when it starts to struggle at setting you want to keep at.
You don't have to match the board brand, just the chip so it should be easy enough to get another either new or second-hand when you do want a boost.
The WD black is a very good drive if a little expensive compared to others (I have 3 at the mo and they have been 100% rock solid, fingers crossed, and they are nice and fast for HDD)
The cooler, yes your better off with a good one but you don't have to spend a fortune. The CM 212 Plus seems one of the beetter and more popular higher performance air cooler at lower price ranges.
That said, you can OC at stock cooler but be warned not to push it too hard as the stock cooler may not be able to keep up with what would be a standard level of OC on a good cooler.
2x4GB of ram should be plenty even for PS. yes more is better but when your on a tight budget even cheap parts aren't justified if not really needed (I would suggest more if you were doing allot and large edits but it's hard to on an $800 budget)