On the low and cheap end:
I have a somewhat old system that handles FlightGear fairly well:
I3-4150 3.5 MHZ
8 GB RAM
Nvidia GEFORCE GT-210
Such a system could be built toaday in the $ 300 U.S. range.
A lot depends on whether you also want to use the machine for serious gaming; in which case you
definitely want an I5 or preferably faster processor.
I've spent a good bit of time watching the task master while running; and have very rarely seen
demand exceed 5 GB of Ram, so think 8 is adequate.
Yes, more video card is better. Today's gamers consider my obsolete GT-210 junk, but it's done well for me. Would recommend a GT-710 or, better yet a GT-1030; however, more powerful cards
require more power supply and better cooling, which escalates cost.
An economical system for about $500 U.S. might be:
Intel I3-8100 4x3.6 (has about twice speed of my 4150)
8 GB Ram
NVIDIA GT-710
The few limitations I've experienced on my system follow:
With Automatic Scenery Downloading on, downloading at start-up can be slow in high scenery areas;
however, if previously downloaded to TerraSync directory, or if area has been downloaded and installed in the Scenery directory, scenery loads quickly at start-up with automatic scenery downloading off. I don't really know if this is a matter of WiFi and Internet connection or processing speed, but suspect the former.
(If it is the processor, doubling speed, as above, would help)
In like manner, with Automatic Scenery Downloading on, I sometimes outrun the scenery and have to "pause" for it to catch up. Not so with Auto loading off and scenery available from
either the TerraSync or Scenery directories on my hard drive. It then works fine.
Best Regards