I can only opt for 1x8GBs, not 2x4GBs.  as mentioned.

 

Sorry but I don't understand what is preventing you from considering a 2X4GB RAM kit?

A 2x 4GB kit is not an option through the online company putting the system together, for some reason. I guess I could email and ask specifically, but pushing the RAM up to 16GBs is not a big deal, financially. Except that it's probably just a waste, no matter how cheap, relatively, it might be.

 

I'll probably go with the 2GB GTX 960..

 

Still undecided about the CPU, i7-4790K or i5-4690K. The difference in price isn't a deal breaker - about £80

A 2x 4GB kit is not an option through the online company putting the system together

Hmm... that sounds a bit dodgy IMHO, perhaps just forcing you to pay more for a 16GB kit.

No, they are fine, I've used them before. I've emailed them to ask why they don't offer this. I guess I can buy the RAM myself and send it to them to fit (I did that with a HDD they didn't have in stock once).

I guess I can buy the RAM myself and send it to them to fit

 

I think that would be a better option. I'm planning on getting some GSkill modules, but I think any of the known brands will give you good quality modules. Also consider the warranty.

 

And re the i7-4790K vs i5-4690K, there will be no benefit getting the i7 for FS9 and FSX. The only think I can think is if Windows10 can somehow make better use of the virtual cores?

The build includes a three-year pick up and repair warranty. The Kingston RAM is guaranteed for life. (My life, or its life, I wonder!).