I don't think any GPU will let you hit 60 FPS with that CPU. Open-world games are very tough on CPUs and this is an extremely old CPU that was already a budget CPU when it was released a decade ago. A GTX 660 won't save you and the GPUs that will both fit in your PC and work with your power supply are going to be less powerful than a 660.
Honestly, to be able to play Fortnite, you're going to need something considerably more up-to-date. Decade-old office PCs don't make good gaming rigs; you'd literally have to upgrade every single part.