There is no better amd cpu, the 9590 and other 9xxx amd cpus are the same exact cpu as your 8320 albeit with a higher TDP and clock. Your 8320 would perfom the same as a 9590 at the same clock speed. Investing money on the now 5 year old AMD AM3+ platform is a bad idea in general in my opinion, I personally refuse to spend the money to upgrade my fx 6350 to something like an 8320 because I just dont believe its worth the money.
I would recommend you wait for AMD zen which should be out in a few months or switch to intel.