Of course there's no real point in buying the 990x unless your board does not support the Xeons going around. They're pretty much the same thing with the second QPI enabled for link. And they're cheap. Like, £60 or so cheap.
I can't remember what Aurora ALX model you had but if it's the 1366 one then the Xeon should work. I know this because I tried a 6 core ES in one and it did boot but after setting it up the screen would go black on reboot. However, this board had two physically snapped pins in it and when I gave it to some one else they confirmed that it did the same with a I7 920 in it so it was the board at fault.
Food for thought any way dude but as confirmed, the 990x will slaughter the AMD FX 8. The IPC of the 990 isn't miles away from Vishera, but once overclocked it really comes into its own, due to the 990 having 12 threads.
Edit. Just spotted you have an R4 meaning not 1366 so I guess you were just asking generally/for some one else.