You need to increase the memory in the PC and yes a faster card would help. However, I would save the dosh and upgrade the whole PC to a quad core amd. Novatech has a selection of motherboard bundles or barebones which come with a case and PSU. Another point, would your present M/B take a new Graphics card configuration and would you have to more powerful PSU? Check the slot.
OnePlus 5T review: Hands-on