Im an avid FSX user and DCS A10, so im familliar with both.
First off, your system should give you good performance in FSX. Just remember, not to push all the sliders to far right. Try it out with option sliders somewhere in the middle and see how it runs, then make adjustmenst from there. Water and Autogen use the most FPS.
As for the complexity of planes in FSX, it all depends on the planes. PMDG planes are very complex and are at very least, equal to the complexity of the DCS A10. Level D is another good one for complexity. There are some good smaller corporate Jets (flight1 Mustang for example) which can also rival the A10 in complexity. The flight models for most FSX planes are good, some better than others. I still get the feeling of flying better in A10 but FSX can do things DCS A10 will never be able to do and vice versa. Your list of addons for FSX is good. Ill also sugest some airport scenery from Fly Tampa, Aerosoft and FS dreamteam. All good stuff.
Hope this helps.
Rob
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Intel Core i7-2600K Sandy Bridge 3.4GHz Oc'd to 4.8GHZ
4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 2000 (PC3 16000)
ASUS GeForce GTX 560 Ti
ASUS P8Z68-V/GEN3 LGA 1155 MB
Sound Blaster X-Fi Titanium Fatal1ty
CORSAIR CWCH60 Liquid CPU Cooler
Rosewill BLACKHAWK Gaming ATX Mid Tower Computer Case
SAMSUNG 27" 2ms(GTG) HDMI Widescreen LCD Monitor