Only if you are gaming on triple monitors is that a factor.
With two monitors, games will be directed to the side monitor, and that takes negligible extra graphics power.
The card you picked is excellent. It is possible that you might score higher synthetic benchmark numbers with dual GTX660, but gaming will be better on a single monitor.
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 or 7990 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 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 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 Maxwell and amd 8000 or 9000 series are due next year.
-------------------------------End of rant-----------------------------------------------------------
As to cases, most any case with two 120mm intake fans or equivalent will do the job.
This antec is not expensive: http://www.newegg.com/Product/Product.aspx?Item=N82E16811129181