120GB for the OS and a limited number of games or just the OS and have your game library on the 1TB drive if you have more than 10 games on your library. If your budget allows, get a 240+ GB capacity SSD.I. In the process of building, phantexs p400 case, msi x370 gaming plus, 16gb of ram?, rayzen 5 1600x, xfx 750 power supply msi cooler, 2 140mm intake fans and 120mm exhaust fan in reat, 1400 exhaust fan at top in rear slot.
Might I ask what your existing system looks like?