I’ve moved over to this more appropriate heading.
Thanks to Dave71k and to Idahosurge and Faildozer for your recent comments  under . “CPU and GPU Hierarchy Charts....” and  “Dedicated FSX spec”
My specs has had a good deserved shake and most of the bits have fallen out
Rather than list yet another updated spec for the forum to view I’d like to ask a few questions and check a few assumptions derived from the comments you’ve already made. They may help those of a similar mind.

As ever the aim is to run FSX (and its successor) at the highest practical settings within a budget of under £2000. Note  No longer “ultimate settings.”
I’m learning but, not enough to do self build. Instead I’m relying on the configitor PCspecialists to set the base spec. They still offers a choice of components and although limited, it’s here where your guidance has been invaluable. I’ll follow up Idahosurge’s suggestion of asking the configitor to consider added other items to the choice.
CPU:-             Sticking with i7 2700k but, overclock to 4 or leave at stock 3.5?  Go for the higher- doesn’t cost much more and it’s done by the configitor. The question is how much difference does it make to running? - which I need to research further. As you’ve pointed out OCing = potential instability, more degrading heat to remove and reducing life, so there has to be significant pros?
RAM:-             My 16GB  is under threat because of its high CAS, higher risk of instability and over capacity. As you’ve suggested down size to 8, keep 1600Mhz  and seek a CAS7 if pos. The prelim Flight spec seeks “6GB for smooth running at high levels”. Will 8 do for both FS’s?  Might the Flight 64bit OS run better with a 16GB (if I can get a Low CAS)
HD(s):-            An SSD or whirly HD. Well both. I like your suggestion of an 510 SSD for FSX and the other for, in my case, HD video editing and Astronomy. That way round because the SSD is faster? But where to put the OS, best for SFX?
COOLING:-       The best of my limited choice is the Coolit ecoII C240 so Ill go with that for the moment, your suggestions noted.
GPU:-              1 x  3GB GTX 580 stays. In my naivety, the bigger the better?
CASE:-            I agree with your better choice and will amend to suite , Coolmaster HAF-X or 932.
MONITOR:-      27” IIYAMA E2773HDS stays . As big as possible  LED HD 1080 HDMI. FSX format is 3:4 not wide screen. I think you can switch to this option on FSX but not sure what it looks like?
I do very much appreciate the advice you’ve given me and value any comments you have.
I couldn’t give the link before because of forum restriction for new members, but I think its lifted now.

http://www.pcspecialist.co.uk/computers/intel-1155-overclocked/

I would not pay them to OC your CPU, you can do it just fine.  Also you need to leave your CPU at default clock settings while you load FSX and all addons.  I would leave everthing at default until you get all programs loaded that you are going to load.  Then you need to let everything run for at least a month, if not two, before you start overclocking.  This way if you have a problem with a program running after you overclock then you know the problem probably is related to your overclock.  Overclocking a PC is a subject that can take a thread of its own so that is all I will say here.  You are not going to degrade your 2600K by overclocking it as long as you do not overclock it higher than it can run safely 24/7.  All chips are different, one can run at 5.0GHz at a certain vcore and another can only run at 4.6GHz at the same vcore.  It is the luck of the draw.  The point is you do not overclock it any higher than what it takes to run it at a safe vcore.  There are other voltages, but I am not going to get into all of this now.

The 6GB of ram spec is from the i7 9XX days, to run at optimum your ram had to be 3GB, 6GB, 12GB, etc.  With SB you need 4GB, 8GB, 16GB, etc. and for FSX 8GB is all you need.  No one know about Flight at this time, but I would think 8GB would be more than enough for flight.

The only problem with putting both the OS and FSX on the SSD is room.  My SSD has 50GB on is, all I have is Win7, Office 2010 and a few other programs that I wanted on the SSD.  I have two drives for FSX, the FSX drive has 110GB on it and the other drive that has nothing but photo scenery has close to 300GB on it.  If you put your OS and FSX on the same drive you will be limited to the amount of FSX addons you can add.  From what I have read in FSX hardware forums other than faster loading times most people do not see a performance difference with FSX on a SSD.  If you want FSX on the SSD then all I would put on the SSD is FSX.  If you think that you are going to get a bunch of addons over time then you maybe better off putting FSX on the HD, but I would partition the HD and put FSX on the first partition.  This is also a subject that can take up a tread on its own.

FSX will let you set the resolution of your monitor.  You just need to make sure that the resolution in Win7, FSX and the Nvidia control panel are all set to the same.  The monitor you show has a resolution of 1920*1080 and that is not a problem setting in FSX.  My monitor has a resolution of 1920*1200.

Regarding overclocking, you need to stress test and monitor your temps while you are doing it.  You should read a couple of guides before you start.  A great hardware site is www.overclock.net and there are a great bunch of people ready and willing to help you out.

What case you get is up to you, what does not look good to one person looks great to another.  What you really need in a case if you plan on a high overclock is airflow.  The two cases I recommend are two of the best in their price range and since I have had them I can recommend them.  Overclock.net has a case forum and if you see a case you like you can post over there and ask the forum members what they think of the airflow.

I’m still checking into the comments you’ve made. Very usefull. Thanks again for all your help.