unless you say otherwise, i believe what you're referring to is the motherboard's CPU TDP threshhold. a "95w" motherboard can only handle CPU's with a TDP of 95 watts, where as "125w" motherboard can handle CPU's with a TDP of 125 watts.
get a "125w" motherboard, that way you're not limiting your choice of processors to put on it.
for example, assuming your're looking at getting an bulldozer (AM3+) system, if you get a mobo rated for only 95 watt then you'd be limited yourself to getting an FX-4100 or 6120 CPU, but if you get a 125 watt mobo then you could get the superior FX-8120 or 8150 CPU's.
You do not determine which size of PSU to get based on the "MB" with one exception, You are using an integrated Graphics unit (IGP).
The wattage rating is generally determined by both the MB and the dedicated GPU.
For a MB with IGP and NO plans to upgrade to a dedicated GPU, a 250-> 300 W PSU is plenty.
For a MB with a Graphic card. What I do is google a review of the Graphic card. They generally show Power under load (Don't use idle load). From that I generally add 20% to get the MINIUM size PSU. You also must add some if planning on (A) overclocking and (B) if Planning to upgrade the GPU to a higher end unit (or adding a 2nd GPU for SLI/Xfire) - Must then take that into account.
ALWAY verify that the PSU is a "QUALITY" PSU, Gppgle reviews for the PSU you are looking at. There are some that DO NOT blong in a compute and are best used as a paper weight!!!