If you're still satisfied with what you're running now, then you may as well save your money until you do a whole system upgrade... my 2 cents

bill1024
I would try to go cheaper. Here is a 980x for under 150$
http://www.ebay.com/itm/Intel-Core-i7-980X-3-33GHz-Six-Core-SLBUZ-Processor-/152708818631?epid=109634758&hash=item238e27a6c7:g:gs8AAOSwqz9ZwG4b
 
There are several for sale right now under 150$
 
Also I would look into dropping an Intel Xeon hexcore, they can be had real cheap and overclock very well.
You may have to do a hard mod to be able to use a xeon.
Find the rev. number and do some research on Westmere mod
 
Xeons that I have used in my x58 systems.
X5660  BCLK unlocked  can be bought now for under 30$ or so
X5675 BCLK unlocked
w3680 fully unlocked Multiplier and BCLK going for 75-80$ or so
E5645 BCLK unlocked

This is the way to go. With a decent SSD youll notice a significant bump in most things.

Not worth it imo. I would put the money toward a new system.

if your 920 is still keeping up with all you need it to then stay with it until you upgrade, the x58 is a great board and i did the hard mod and run a w3680 in it, its pretty quick but it still is showing its age, dropping 150~200 just on a CPU would not be worth it, there is much newer tech in todays boards, one big one is storage speed and options, the x58 760 (the same one i have) is sata 2, usb 2. hard to get working with pcie storage and that right is what will make your system "feel" fast, i would just wait and build anew, if you need the extra cores and unlocked cpu the grab the w3680, it basically the 980x with a xeon name. you will most likely have to mod the board, search in here and there is a thread about doing, its pretty easy to do.

After a bios update, I upgraded my 1366\ I7 965EE  to a Intel Xeon X5690 CPU. It was absolutely worth it. And you can get them for cheaper than the 980 just a tad over a $100 right now on flea bay.
Easy to overclock it sits @ 4.165 with two more cores and hyper-threading on np at all. the 965 hated hyper-threading on anything over 4.0. That 1366 was the best investment I ever made,
as far as upgrades I dropped a 1080 ti in that puppy even with the pci-e 2.0@16 its still kicks arse.
Theres me 2c
good luck

My configuration is similar - E760, started on a 920 D0 and bought a 990x when they dropped in January of whatever year it was (2011?).  Anyway, the thing was a BEAST to have around when folding really benefited from -SMP, Overclocking, and gave the huge bonus points for -BIGADV.  Once they edged out of 12 Threads and required 18, it became less useful.  I still use it today as my main system (the bones of which are circa 2009!) and I don't have many complaints - except the lack of USB 3.0 Support, the lack of SATA 3 (6G) support, the lack of an M.2 slot, the lack of PCIe 3.0 (this board is 2.0 spec).... 
 
So I personally wouldn't drop the extra $ on the chip unless you're doing something CPU intensive (running VMs, Encoding video with CPU cycles instead of GPU, or running something like Cinema 4D / NetRender).  I don't think you'll notice the bump in performance... although the picture of 12 threads when you hit CTRL-ALT-DELETE is pretty cool.  

Just my opinion, but the  unlocked multiplier in the w3680 makes it a better buy than the x5680, x5690. However, you will see a noticeable difference in your system with the x5690 upgrade.