If you know what software/applications you plan to use your PC for then you can look specifically at the online benches for the FX-9590 and whatever I7 model you may be considering. General benches may not provide an accurate performance indicator for what you use your PC for so the better that you can quantify your typical usage, the easier it is to determine what benches reflect the activity you do with your PC.
For most people cost/value is also a factor so after you narrow your choices be sure to check current prices from whatever source you will be using to supply your CPU.
Since 2008 you have tons of i7s in diferent generations, and architectures...But if you refer to the latest i7s from haswell and skylake matching up FX9590, you need to also understand that they have difrent architectures and they are strong on difrnt things. FX 83xx and 9xxx have 4modules with 2cores on each module, wich means 2 cores kinda share cache memory capability. For Intel Haswell&Skylake they have 4modules with 1core for module and each Core Hyper Thread to 2, its kinda 1 core splits itself in 2 loosing half performance. Thats why i7 shows 8 threads on Windows.
Intel cpus have powerfull single core performance if not using Hyper thread. Therefore, if you compare na i7 with an FX on single core performance tasks FX will loose, however on multithread/all cores performance FX matchs-up i7, because of FXs have 8 real cores computing VS Intel 4cores- HT to 8threads. Intel i7s also wins on cache memory speeds read/write, but for gaming its not relevant.
In my opinion the strong of FX cpus is the price/performance gain and the fact their are unlocked CPUs. For Intel they are much more expensive but their architecture also allows PCI-e lanes directly conected from CPU to the GPU to have fastest data transfer. If you refer to an K- unlok i7 you have to pay even more, but they do the same but can run on higher clocks.
All are great chips thats for sure