Memory Suggestions for New Build

Go for 1866 MHz and if your mobo supports quad channel go for 4x4gB cards. I would have suggested 4x2gB but as 2gB cards are rare these days go for 4gB sticks.
8gB is enough but try to fill all,the slots to reap the benefits of quad channel. In this case you have to buy 16 gB and donot think of upgrading later. Because when you buy the cards later they will probably have different version, and will cause a lot of crashes.

Also get a ram cooler if you choose to fill all slots.

I was thinking of getting G.SKILL Sniper Series 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1866 (PC3 14900)
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Is this a safe bet?

Or maybe these
G.SKILL Ripjaws X Series 16GB (2 x 8GB) 240-Pin DDR3 SDRAM DDR3 1866
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

How much does the CAS effect performance?

I'm sorry your mobo is dual channel. As for latency, less latency means faster data handling. Example 8-8-8-20 is better than 9-9-9-24.

You can get either but there is no speed increase for the 8gB cards just that you will have more memory.

In general the 8gB cards will be slightly slow even they have same frequency and timing.