It is hard because each of those games is different when it comes to the hardware they rely on.... GTA and Trainz hammer the CPU and the Hard Drives to no end so you would need a fast CPU and a fast HDD(SSD preferably), while Flight Sim slams the CPU with just enough forced down to the GPU to get by. while Farm Sim and railsim do a decent job at distributing the load fairly even, which in turn give it the best performance even in areas densely populated....
My most recent specs are Windows 7 Ultimate SP1 64-bit, AMD Phenom II X4 970 3.5GHz, 16GBs Kingston Hpyer X Limited Edition DDR3 1600, MSI 870A-G55 with USB 3.0 and SATA III support, EVGA GeForce GTX 660TI SC 2GB, Rosewill Lightning 1000W PSU, Creative Sound Blaster Audigy Recon3D Fatality, Kingston Hyper X 3K 120GB SSD, OCZ Synapse Cache, OCZ Revodrive 120GB, Two 1TB Seagate Momentous 7200rpm, One 1TB WD Green, One 2TB Seagate Momentous 7200rpm, Corsair 900D case(with 14 120mm fans), Razer Naga Molten Edition gaming mouse, Razer DeathStalker Ultimate gaming keyboard, Three 19 inch displays using Nvidia Surround.
And it runs all games at max settings hovering around 35fps to 50fps pending the game and exact scene(exception to the rule is trainz which can barely squeeze out enough performance to be even playable even on an SSD) Final cost is most likely somewhere around 2200USD(not counting keyboard, mouse an screens would be near 3000USD with them), with a upgrade to the Mobo and CPU coming shortly.