But your PSU is fine. It looks worse than it actually is because it is a single voltage one.
And if you worry about the PSU, why not get a Powermeter in the next hardware store.
Here in Germany you can get something decent for like 10€ or so. That should show you that your system is like 150W or so primary with the stuff. So the 240W PSU is more than plenty as the CPU has officially a TDP of 95W but the GPU also is limited to 75W...
The Rest rarely does matter so 20W or so we#re talking about here.
That means that we're talking about 190W at worst, in theory.
In Gaming loads it will be much less...
I can understand your worry and your desire to get something better. But you should also think about the next system. What happens when the CPU dies. The Board you will be getting dies. Stuff like that...
Because we are talking about pretty old stuff that, back when I was young and beautiful, was hardly usable at that age. Thus cheap...
But today, for whatever reason there is a high demand for those old things, so the prices are stable or rather increase even.
The ASUS Z77 Extreme 4 you are mentioning is a ~100€ Board - NEW from a Dealer at the time! Or rather 120€ in June 2012...
https://geizhals.de/asrock-z77-extreme4-90-mxgkx0-a0uayz-a746839.html (click on "Preisentwicklung" or that diagramm symbol)
So you see why I think the Price should be around half of that or even lower!
So I'd spent as little as possible on the Motherboard and get the cheapest one I can find that looks OKish that can run your CPU.
For example:
https://www.ebay.de/itm/PEGATRON-IPMSB-QS-REV-1-02-Q67-2x-USB3-DVI-Sockel-1155-m15122/273638558963?hash=item3fb6210cf3:g:~0QAAOSwu4dcFf9t:rk:4:pf:0
Or something from your region:
https://www.ebay.de/itm/Dell-WR7PY-CN-0WR7PY-Optiplex-7010-SFF-Socket-1155-Q77-DDR3-Motherboard-No-BP/382709581731?hash=item591b44c7a3:g:vPAAAOSweiFcLjVU:rk:4:pf:0&LH_BIN=1
Look for similar things in your region. And ask if your 95W TDP CPU runs on it!
Many times with OEM Boards they are limited to 65W...
but be aware that those Boards have specific layouts so it is a bit more pain to get them to run in a "normal" Housing"
And they often come without I/O Shield for whatever reason...
With that said, that should leave you with 150€ or so for the Case and PSU - wich means you can get some really nice things there!
Or look for older OEM Cases as well. For example FSC Celsius M460. Yes it is ugly, but it is a good case...
