Components for new computer (motherboard especially)

The "K" allows you to overclock. You can expect about a 20% improvement if you overclock.
Few games use more than 2-3 cores, so the extra hyperthreads of the i7 chips are largely useless.
I Save some money and use a 3570K or 4670K instead. Use the $100 saved elsewhere.

You will want at least a $30 cpu cooler like a cm hyper212. I would avoid the liquid coolers.

No game, by itself will use more than 2-3gb. 8gb is the norm. I like 16gb if you are not budget constrained. Windows will keep more code in ram available for instant reuse.
Buy a 2 stick kit of ddr3 1600. Faster does not help much.

Gaming performance is primarily driven by the graphics card.
Use the $100 saved to buy a GTX770 if you can. Buy only a quality psu brand like Seasonic, xfx, antec, corsair...
A 600w psu would be appropriate for a GTX770.

Motherboards have decent HD 7.i sound. A separate sound card is not needed.
Motherboards will include a lan port. If you need wifi, then you will need a wifi card.

For a 3570K, buy any Z777 based motherboard.
For a 4670K, buy a Z87 based motherboard.

Do not plan on dual cards.
Here is my canned rant on planning for dual cards:
-----------------------------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 gtx690,7990, oe R9-290X is about as good as it gets for a single card.

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 GTX780 only needs a 575w psu.
When you add another card to the mix, plan on adding 150-200w to your psu requirements.

Even the most power hungry GTX690 only needs 620w, or a 7990 needs 700w.

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 gpu's 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 gpu support is dependent on the driver. Not all games can benefit from dual cards.

e) dual cards 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 Maxwell and amd 8000 or 9000 series are due next year.
-------------------------------End of rant-----------------------------------------------------------

Cases are a personal thing, and most will do their job.
Look for one that appeals to you; you will be looking at it for a long time.
Look for at least 2 120mm intake fans or equivalent in exhaust.
$50 will buy you a perfectly functional case.