sorry for getting u guys confused. 'heat' is not only concentrated on one section of the system. as i've said, ideally, POWERin = POWERout. If you have 300w Power source supplying a 250w load (CPU,HD, accessories etc.), the 50w excess will be dissipated as 'heat' or 'waste'. on the other hand, large-wattage PSU also have large transformer cores and heatsinks that helps stabilize the heat dissipated.
Although its also true that you 'can' have a system running on higher wattage yet cooler compared to another system running on a lower power rating, this is because the latter may have lower efficiency rating compared to the other. Try doing the same test on both PSU with different power rating but same efficiency factor, you will notice the difference in the temperature right away.
anyway, it all boils down to the efficiency of the PSU being utilized. On the other hand, why pay more on using 450W on a 250W load when you can save precious bucks when using just 300W?

just a thought... (omg...this is so 'out of topic')