Let's see . . . got a good deal on a 940, will not OC, willing to go SLI. Here's some suggestions:
Gigabyte ATX mobo ($210). Pefrectly servicable, recommended, and suitable for OCing as well. Since you have chosen your case (not a bad choice either) and are willing to SLI, there's no reason not to go with an ATX mobo:
http://www.newegg.com/Product/Produ...87&IsNodeId=1&bop=And&Order=PRICE&PageSize=20
GSkill 3x2GB DDR3 1600 CL8 memory, $150:
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231247
or, if you want to save $20 (and not notice the difference in speed in real world applications lol):
GSkill 3x2GB DDR3 1333 CL9 memory, $130:
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231223
That's all you need, and answers the questions you asked.
[Edit: I missed that you already own the 260s. My Bad.]
However, I'd like to offer a suggestion if you are buying 2x260s at retail prices. Don't lol.
1xGigabyte GTX 460 (or buy 2 if you are gaming above 1920 screen resolution), 1GB, OC'd, ($230)
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125333
At stock, the performance of the 460 and 260 are roughly comparable:
http://www.tomshardware.com/reviews/geforce-gtx-460-radeon-hd-5570-gaming,2697-7.html
One should suffice at 1920, buy the second if/when you need/want it. For a few bucks more you get DX11, much lower power consumption, and far less heat.
Have fun!