those games doesn't require much performance to run.
i'll say minimum 4 gig of RAM. A CPU that is not from the stone age. (you might as well get a quad core. Check out some core i5 CPU's. any of them will do just fine handling those kinds of games)
Now for the GPU:
because ether truck simulator and minecraft require that much, i'll say
Nivida Series 500: don't go lower then the 550ti
Nvidia Series 600: don't go lower then the 650
AMD series 6000: don't go lower then 6790
AMD Series 7000: don't go lower then 7770
i'll personally go fo Nvidia 660 or an 7850 if i were to buy a PC to only handle truck simulator.