This is just a question. If he steps down to a lower PSU now and wants to up grade a lot more in his rig. Shouldn't he now buy a bigger PSU to cover his up grades for the future? This is just a question for my own curiosity.
well, yes and no. if your rig only needs 400W putting 1000W psu in is kind of a waste (even though it's not working as hard the turn down efficiency is questionable). plus some voltage regulators have trouble dealing with such huge PSU, you're more likely to see heavier noise values and have worse results when you try to OC anything.
I'd say if you plan to upgrade to a point where you need 1kW psu (and as you point out you want to go big ahead of time), make sure the mobo is made to handle that kind of power.