Best OS for specifically one video game and Chrome?

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What would be the best OS in terms of performance for a rig that will be running specifically StarCraft II and Chrome accessing very simple sites such as nios.kr and PlayXP?

I will just be installing Chrome, NVIDIA and ethernet drivers, and StarCraft II. Nothing more.

Specs:
Pentium G3250 @ 3.2GHz
GTX 650 1GB
4GB DDR3 RAM

 

[esco_sid]

windows 7 or above