No siginificant changes could be seen if you reduce the maximum power rating of the PSU.
The PSU will draw power only as needed.
Some efficiency changes for under 50% load and near 100% load do exist but not really significant.
You won't need to replace the PSU because of that.
But you will perhaps need to replace the PSU if it is too old, due to the capacitor and transistor aging...but looking at your specs...the PSU is less than 4-5 years old..
So...no replacement is really needed.
If you wanna reduce the power consumption, the best way is to buy new low powered procs, etc. (components)...but not really necessary...
Buying new parts won't really save you enough. I'd just underclock/volt what you already have. Atom doesn't perform that well anyways. Find the highest stable speed you can with a Vcore of 1.1 or 1.2V.
I would evaluate the cost of Servers and components versus a simple NAS. I have 11 PC's in my home (7 in home office, 4 for mom and kids) being 'served" by this little thing for the last 4 years. Kids use the built in i-Tunes server and keep movies there but personally, I just use it for data storage.