These days, even on an old/outdated system, you should at least have 4-6 gb of FREE RAM - and a dedicated graphics card with at least 1024gb of VRAM - these specs would have been common a decade ago.
In roughly 12-24 months from now, future FlightGear versions can no longer be expected to work on such systems.
Again, keep in mind that a workstation/server-based system will typically offer plenty of RAM (64+ gb), plenty of cores (12+) - for under $300 US, all you'll need is add a dedicated graphics card (given the current situation, I would also opt for a GPU not much newer than 12-18 months) - but even just spending between $100-200 US should be enough for most people's needs, and you should still be well under $500 bucks.
A while ago, I purchased a refurbished/used HP Z820 with GPU (GTX 1060 6 GB) and 192gb of RAM for a little under $900 bucks + shipping.
And given FlightGear's architecture, it's unlikely that FlightGear is going to be able to fully/properly utilize such a system over the course of the next 5-10 years (but we will see)
Either way, there's no need to spend a ton of money on dedicated gaming rigs, some outdated CAD hardware (workstation or server) will typically suffice - having plenty of RAM helps obviously, and adding a dedicated GPU is comparatively simple and cheap. Besides, rebuilding FlightGear from source is possible within a couple of minutes, thanks to all the resources.