Running very smooth at Ultra Settings on my system with following specs:
CPU: i5 2500K @ 4.8Ghz
GPU: AMD r9 290 @ 1150/1350
RAM: 8Gb DDR3
The only thing i notice is, that almost every alpha game is silly optimised (due to their state of developement), so they drain a lot power from your CPU and not GPU.
Upgrading to a newer CPU with a better thread handling etc improves way more than just clock speed.
Cause i got at least two alpha games, where my CPU is on limit and my GPU idles around, resulting in a 200+ framerate and still a laggy or slow gameplay. (a newer CPU like Xeon with 3.4Ghz handles the same situation with a way smoother gameplay)
So if you consider in playing alpha games you may think of getting a newer generation of CPU.
Else every new medicore CPU will do fine and should be enough for every finished game.