Depends on the purpose of the PC. For the average home user it is not worth the cost. For gamers, the money is better spent on the graphics card.
However, for 3D rendering, video production and financial modelling, that's a different story.
A quad core is good for FSX now and I thought it will also be ok for the new reboot of the sim but I was wrong. MS is now recommending a hex core. I don't know whether it will be wise to build a hex core for MS Flight.