In the end this come down to three things. First any computer running less then 8BG or RAM is still almost lost in the modern era of gaming. Second., you need a good ATI or nVidia graphics card. and third a processor over 3 Ghx.
I am not going to put a list of what to buy. because i have even seen high end computers get chocked up on some of these flight sims.
Hell i have an older rig in my office, and it still runs just about everything flight sim i have.
It has FS9, FSX, CFS2, P3Dv3 and P3Dv4 aboard it. Tons of payware aircraft for all of them. And well i am getting good framrates with good graphics. Hell the system here is the following.
ASUS P5Q Pro
Intel Q9650 Quad core processor over clocked to around 3.5 ghz
8 GB or RAM
ATI Radion HD6890 series video card.with I GB GDDR5 video RAM. (This card still holds its own even in modern shooters)
You don't have to spend tons of money to get a good system up and running to handle any of our flightsims. You just have to remember to remember the three things. Good RAM count of 8 GB plus of RAM, good video card that can support DX10 or higher with a good VRAM count, and a processor set up to run over three Ghz.