My opinion....

Mid tower case is fine - that's all mine is, and ever have been, and I have 3 drives (no SSD's) in it.
Liquid cooling is good for if the temperature gets to  the mid 80 to 100+ degrees F during the summer! I have the Corsair H80I, can't see anything wrong with the Asetek 510LC. I don't really know much about it, because I never looked further than Corsair who I recognized and I had known them to be of good quality. I did search the internet to be sure there was nothing bad reported about the Corsair H80i. It's always good to search the internet for anything bad about what you are interested in getting for yourself.

i5 4670K is good and is good for overclocking - another good reason for liquid cooling for more GHz

At least 8GB RAM. I have 16GB and am looking into another 16GB, but 8GB is good enough on a 64bit OS for FSX.

2 HDD's are fine,  more space for less price than SSD's, but SSD's are rocket ships! HDD ARE slower, but I'm okay with that (my personal choice). FSX would be best on (a) a completely separate HDD than the one that Windows is installed on, or (b) 1st partition of the second HDD, or (c) another partition (preferably the 2nd partition) on the main HDD, or (d) the root C:\ (it IS okay too, but not as good as a, b or c). Be aware that each new partition is towards the outside of the drive and so can be slower (1st partition has a faster retrieval speed than the 2nd partition and so on). The further from the centre of the disc the heads get, the longer it takes to retrieve the data. It is always good to defrag the FSX partition/drive on a regular basis. NEVER defrag an SSD!!!

I'm not into ATI cards (that's my personal choice), but modern ATI's are cheaper and fine for FSX. or so I believe. Others who have them would be better able to give you the info on them. Otherwise I would recommend a GTX 660/670/760/770 more or less in that order. Newer is better , so 700 series is better IMO. A 2GB GTX660 (OC) sell for about $AU110 less than a 2GB GTX 760, so the 660 is the budget option. Strangely enough, my 2+ year old 1280MB GTX470 is performing very well on my system and even with some of the newer games. This can be the biggest threat to your budget. There is a thing about having a great system with a not so great graphics card, that will cause a performance drawback. The other side to this is getting an great GC for a mediocre computer, you'll never get the full performance of the GC under those circumstances. GTX 660/760 (or the equivalent ATI cards) should be perfectly fine.

CrossFireX (ATI) or SLI (Nvidia) are of no use for FSX, or so I have heard, but seeing as I've never done it, I wouldn't know. However, IF you are into other games, make sure you get one that matches the ATI or Nvidia card you get if you plan on doing it somewhere in the future! My Gigabyte Z77X-UD3H motherboard has both, but it cost $AU275 ($US283) at the time I bought it.

For the power supply, Corsair or Thermaltake are the best (IMO). I suggest you look for a 750 or better to be on the safe side for power needs. Mine is a Themaltake Evo Blue 750 watt PSU, with plug-in leads for the HDD/DVD/GPU connections, which keeps the case uncluttered with useless unneeded cables and provides a better airflow through the system to help keep it cooler.

Bruce is right. The AVSIM Hardware forum is great for getting more info, both good and bad, on the systems out there. Even better if you find people who have the same/similar system as you are planning to buy.