To answer your questions and my thoughts:
1. A 7850 requires as little as a 450w psu. But, do not skimp on psu quality. Here is a list of psu's sorted by quality tiers:
http://www.eggxpert.com/forums/thread/323050.aspx
It is not wrong to overprovision the psu a bit, say to 550w-650w. It will draw only the power it needs.
2. I doubt that added vram is worth the price.
Here is one study: http://www.pugetsystems.com/labs/articles/Video-Card-Performance-2GB-vs-4GB-Memory-154/
3. It is easy to add fans later. Your parts are not overly hot, and tolerate heat well. Wait and see how you do.
4. Z77 based motherboards are usually not more expensive than Z68 and offer added capabilities, like 6gb sata.
Most any will do. Consider a M-ATX motherboard, they are usually less expensive. They only have 4 expansion slots vs. 7 for full ATX. But really, how many do you need? For most of us, only the single pci-e x16 slot will be used.
5. No game uses more than 2-3gb by itself. 8gb is plenty unless you will be heavily multitasking or running 64 bit enabled apps like photoshop. Ram is cheap.
Speed is not of much importance. Buy a 2 stick kit of DDR3 1600 low profile ram.
6. You bought a "K" so you can overclock a bit.
Buy an inexpensive tower type cooler with a 120mm fan. cm hyper212 or Xigmatek for example.
7. I will never build without a SSD for the os at least. It makes everything you do much quicker. A hour windows install takes 10 minutes.
A 120gb ssd will hold the os and half a dozen games. With 240gb, you may never need a hard drive at all. Perhaps defer on the hard drive initially, and add one for storage whenever you fill up the ssd.
8. Do not bother with sli. Here is my canned rant on that:
-----------------------------Start of rant----------------------------------------------------
Dual graphics cards vs. a good single card.
a) How good do you really need to be?
A single GTX650/ti or 7770 can give you good performance at 1920 x 1200 in most games.
A single GTX660 or 7850 will give you excellent performance at 1920 x 1200 in most games.
Even 2560 x 1600 will be good with lowered detail.
A single gtx690 or Titan 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 GTX660 needs a 430w psu, even a GTX680 only needs a 550w psu.
When you add another card to the mix, plan on adding 150-200w to your psu requirements.
Even the strongest GTX690 only needs 620w.
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.
c) Dual cards do not always render their half of the display in sync, causing microstuttering. It is an annoying effect.
The benefit of higher benchmark fps can be offset, particularly with lower tier cards.
Read this:
http://www.tomshardware.com/reviews/radeon-geforce-stutter-crossfire,2995.html
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.
The GTX780 and amd 8000 series are not that far off.
-------------------------------End of rant-----------------------------------------------------------
9. Take the time now to download and read, cover to cover, the case and motherboard manuals. You will learn much.
-----------------good luck---------------