If anyone is still interested on how to build a FSX pc, I got some answers for you.
First of all: THERE IS NO PERFECT FSX PC. So you can throw those hopes overboard.
all prizes I listed are in US dollars, as I did my shopping mostly on US sites Ok, here we go!
I recommend you DON'T buy a pre-fab PC. 99% of the time you pay way more then the parts are worth. If you don't feel comfortable with a pc build yourself, KNOWLEDGE is the key! Building a PC these days isn't harder than building your average IKEA closet. It's almost completely idiot-proof. You can it have it up and running in under an hour. So it's worth to do some digging, as you can save 100's of $. So you can spend those on better hardware!
So let me familiarize with the components of a pc:
There are 8 crucial components and some other "accessories".
1. CPU
2. Motherboard
3. GPU
4: Memory
5: Storage
6: Power supply
7: Casing
8: Cooling
Optional:
Optical discdrive
Soundcard
Wireless Internet card
Extra storage units
SSD
Since you've come to this topic I'm assuming you know what these things are and do, so no further explanation about them.
First of all, FSX benefits the most of a good CPU. To make it run smooth you should go way over 4.0Ghz. Around 4.5Ghz, things should get real interesting. This is a kind of magic number to simmers. Everything above is only better. But 4.5Ghz should do everything with flying colors. Intel CPU's are the best ones for FSX, AMD's are cheaper, but they will perform significantly worse in FSX. This might not be the case for other games, but since you are building a PC for FSX, you should be focused on what gives the best performance for FSX.
Recommended CPU: Intel i7 4770k. The 'k' stands for that it can be overclocked. Something you definitely want, because the baseclock is 3.5Ghz.
NOTE:Even though FSX doesn't take benefit of Intel's Hyperthreading, which is in the i7-series but not in the lower i-series, some say you can better buy an i5 CPU as it costs way less. Don't do this as an i7-processor has a much larger cache, where FSX benefits from. When an i5 and an i7 are compared (with the same Ghz) you'll get better performance out of an i7.
Choosing motherboards is some of the confusing things to choose in a PC. If you are shopping for one, make sure you get one that is compatible with your CPU. Otherwise it will be of no use. And without a motherboard you only have a casing full of components that you can't link together. Also, make sure it's suitable for overclocking. Otherwise you might destroy it in the progress along with the rest of your PC.
That FSX benefits the most of a good CPU, doesn't mean you can save money on a GPU. A high clocked CPU combined with a cheap GPU will create a bottleneck and performance will decrease significantly. A good GPU for FSX comes from nVidia. ATI cards are great for gaming, but they have drivers that don't support FSX very well. So avoid AMD-cards and go for the nVidia.
Recommended GPU: nVidia GTX 780. While this one is really expensive, you can go for a GTX 770. Make sure you don't have a standard card, if you go for the GTX 770, go for one that's already overclocked. MSI, ASUS or EVGA deliver high quality GTX 770 cards that are nearing the GTX780, but for $150 less.
NOTE: If you want to use more than one monitor, you should go for an 3GB or higher model.
NOTE 2: Never and I mean NEVER, go for dual GPU in SLI. It will reduce FSX-performance immensely. And it costs so much more.
Go for at least 8GB. Since FSX is a 32-bit application, it can only use up to 4GB or RAM. If you are running a 32-bit pc (which I don't recommend) there's no need for more as Windows 32-bit also can use up to 4GB. But when you are using a 64-bit version of Windows, go for at least 8GB. This way you have maximum RAM-capacity for FSX, and 4GB's left for the OS and other programs (such as REX or Active Sky) I recommend you go for higher clocked models. 1866Mzh should do fine and everything above is a nice bonus. Make sure you check what kind of RAM is compatible with your motherboard. Some motherboards require quad channel RAM. This means you need 4 RAM modules. If your motherboard is dual channeled, two modules will do.
No real recommendations go here, go for the cheapest and fasted you can getStorage is very important as it speech for itself. FSX-performance will increase if it gets an seperate HDD or SSD. So you might end up with 3 different storage units. One SSD for Windows (the start-up time for your PC is only around 20 seconds, even when the SSD is full). One SSD or HDD for the rootfolder for FSX. An SSD will reduce loadtimes in the sim immensely. And a big HDD for all your files. Note that the prices of SSD's are very different. Go for the cheaper ones from quality brands like SanDisk, Kingston or Intel.
This is something you don't want to underestimate of it's importance. A powersupply that does not give enough power to your pc components will ruin your PC overtime. Also, make sure you get one from a good brand like Corsair or CoolerMaster. A cheap powersupply of 650W can often not go further than 450W. So don't cut back on your powersupply, as it can ruin your complete PC! You better have a powersupply that gives more power than you need, so when you upgrade a component, you don't have to buy a new powersupply.
Cooling is one of the most important points of your PC. A hot PC will have more problems than a cooler PC, so beware of this. When buying a CPU, there is a coolant supplied, but this one isn't capable of cooling the cpu over it's baseclocking. When you are going to overclock (what you really should do) you need a new cooling-unit. When you stay at 4Ghz or less, an air-cooler should do fine. But beware that air-coolers are pretty heavy, they stress your motherboard. This can be deadly when you order a pre-fab PC with an air-cooler. Because an air-cooler is only attached to the motherboard, and has no supports whatsover, it can begin to wobble during shipping, which can, in worst case, crack your motherboard. Go for an watercooling system instead. You might think that you don't want water inside your PC, but there are closed-circuit watercoolers like the Hydro-series from Corsair. Which are practically leak-proof. Also, when you go over a CPU clockspeed of 4Ghz, there isn't really an alternative. Also here is cutting costs not a good idea. A high end watercooler goes for around $100 (Corsair Hydro H100i) and will do the job way better than a high end air-cooler (around $75)
Recommendation: Corsair H100i, great value, it is a bit big, but it will do an outstanding job keeping your CPU cool.
Last but not least, the casing: When shopping for a casing make sure you don't go for visuals only. Make sure the airflow in the tower is good, so your parts keep cool. Also, don't forget you have to put everything in here, so it's better you buy a too big one, than a too small one.
For an example, here is a list of what I just ordered last week. (Still no sign of it coming my way -sigh

)
This is my setup I just ordered:
CPU: Intel Core i7 4770K
GPU: EVGA GeForce GTX 770 Dual SuperClocked ACX 4 GB
Memory (RAM) Kingston HyperX Beast 8 GB : 2 x 4 GB
Case: Aerocool XPredator - Evil Blue Edition
CPU Cooling: Corsair Hydro Series H100i Perfomance Liquid CPU Cooler
Motherboard: ASRock Z87 Extreme6
Optical Disc reader: Samsung SH-224DB
Power supply Corsair RM Series RM850 80 PLUS Gold
SSD: SanDisk SSD 128 GB
HDD: Seagate Barracuda 2 TB
Monitor: Acer G276HLAbid LED (TFT 69,0cm (27") / 100.000.000:1 contrastratio / 2ms respondtime)
I hope you learned something and find this guide useful

. I can't say it enough: "Build your own PC!" You'll save money big time!
