What motherboard do you have?
What do you plan to use the PC for?
If gaming, what kind of games do you play?
FX 8 core needs a little beefier VRM and good cooling. And they are good for well threaded games. I'd suggest an 8320E or 8350. The only time I'd ever get the 9590 is if you can get it on sale, like I did, for just $30 more than an 83xx series.
Most modern games and high end GPUs will do just fine with an FX 6 core for gaming. And the 6300 and 6350 OC nicely without generating the heat/voltage stresses of the 8 core.
Example:
https://www.youtube.com/watch?v=4ZHlo0DAI38
And no, they aren't as power hungry as some suggest. With a good board, good cooling and clean power you can often tune them to run at stock at much lower than default voltages, or OC nicely on stock voltage.
Examples from my builds:
FX 6300 #1 4.4GHz on stock voltage, Gigabyte 970-UD3, no LLC
FX 6300 #2 4.5 GHz on 1.375v, Gigabyte 970-UD3P, Medium LLC
FX 6350 4.5GHz on 1.3625v, Gigabyte 970-UD3P, Extreme LLC
FX 8350 4.5GHz on stock voltage, 990FX UD3, no LLC
FX 9590 4.9GHz on all 8 cores stable at 1.40v, High LLC. Yes, 1.40. 5.0GHz on all 8 needs 1.475, and yields minimal gains, so for power savings it makes sense to run it at 4.9 instead.