I know some older games like fallout 3 only use 1 core. Will this cause issues if im using an fx-8350?
no the game will just use one core.
I have to agree with the poster above me. The 8350 is a solid processor. I recently built a gaming PC for my cousin using it and a 7950 and I had no problems at all playing anything from the original Starcraft to Battlefield 3.
thats what i use and i play any game without issues, i play css often and thats as old as ive gone and no issues to be had
the only issue you will hit is that some old games dont really like newer videocards/drivers, in those cases, use google to find help with your problem.
also note: its even worse with intels video....the drivers are only tested with and optimized/fixed for current titles, and then, only for titles used by top bench sites...