The graphics card is fairly new, but those other three things are about 4-5 years old. I have 400$ that I would like to spend on upgrading, and initially I was going to buy new motherboard and CPU, but since I started doing some research, CPU's today are about one more GHZ and that's about it, not to mention that along with new CPU i would have to buy a new motherboard because the socket is apparently ancient.
Wouldn't an 120GB of SSD and 16GB of RAM be a much wiser and cheaper buy in terms of overall performance?
By the way I should add that I use this computer mainly for gaming and occasional rendering.
As far as gaming alone. An SSD wont help you alot. It will make your pc boot faster as well as operate a bit quicker, but while actually in a game the performance wont be noticeable. As for the RAM, 8 gigs should do you fine, 16 is a bit overkill. I think your best bet would be the CPU mobo upgrade. Your GPU seems good as well. I run a 670 which is just a hair over the 7970 and it eats just about everything I throw at it (1080p).
Clockspeed doesn't dictate performance all the time. Look at Pentium 4 for crying out loud, a 3.6 ghz gets beat by a puny disabled core processor at 2.2. This sort of jump in performance happened after nehalem, specifically sandy bridge is still viable today, while nehalem is showing it's age (your 920 isn't much better than a q9550). This is an informative benchmark http://www.tomshardware.com/reviews/ivy-bridge-wolfdale-yorkfield-comparison,3487.html
You should get a new i5 and call it a day. I'd get a xeon 1230v3 because it's an i7 without integrated gpu at the price of an i5, and a cheap motherboard like a ASUS z87/97-K or A . Or maybe the i5 4690 (k is unlocked for overclocking through multiplier, get a proper cooler if you plan on going this route)
Why Haswell?
It's quite a bit faster than your current setup. A ssd would be nice if you can afford it. You won't need more than 8gb for gaming and occasional rendering. If you're rendering everyday then sure, go for 16gig.
CPU power is about more than just GHz , it also depends on other things like process size. The 920 uses a 45nm process vs the current generation Intel CPUs use 22nm process. I am assuming if you are looking for a new mobo and CPU in the $400 range then you are probably looking at the i5 4670K or something close to that. Comparing the 4670K vs the 920 you see pretty significant increases in performance with a decrease in power consumption.
In single-threaded tasks, the i5 has 63% higher performance.
In multi-threaded programs, the i5-4670K microprocessor is 22% faster on average but the difference in performance is application specific. In some of them the processor could be up to 44% faster.
Power consumption drops from 130w to 80w
Overclocking is super easy on the i5 as well
As UnHolyKing said, the SSD will improve boot and load times but the in game experience will not really be impacted and 8GB of RAM is still more than enough for most games. I would recommend going with the new mobo and CPU.
Here are the links to the comparison tests for the CPUs http://www.cpu-world.com/Compare/456/Intel_Core_i5_i5-4670K_vs_Intel_Core_i7_i7-920.html
Pretty decent hardware still so I'd ask myself if I found that the system felt slow in any perticular situation.
But get an SSD and don't be too cheap. 240-250GB is affordable now(look around for prices and notice how smaller drives have almost become irrelevant) and if you haven't had an SSD before then you'll be surprised how much it can do for the overall performance of the system. Even when running games like Battlefield 4 I would be annoyed at the load times in between maps without an SSD. And others would be able to start playing the new round before I even got in.
Every system you use more than briefly every day deserves an SSD.