While it's nice to have the OS on an SSD (say 256GB, avoid 120/128GB if budget allows), that just gives you faster boot and general performance. Most aspects of gaming are just as good from a HDD, say a WD Black.
Prebuilt or are you willing to build?
Have you already set a budget and/or selected any components yet? I'll send one of our gaming build specialists to give you some advice -- in the meantime please provide a budget and any details of specific stuff you have or are looking at already. Thanks and welcome to Tom's Hardware.