If you know the motherboard model and the CPUs it supports, you can look up their power requirements
here.
But frankly, I would not bother because a new CPU is a big investment that is highly unlikely you can carry over to a new computer when that time comes.
How much RAM does this system have? Typically, in terms of performance you get much more bang for your money by upgrading the RAM (depending on how much RAM at the start). And adding RAM rarely puts a significant increase in demands on the PSU that would require a new PSU.