I would only upgrade the power supply if you need to. Don't buy a big powerful power supply unless you are going to use it, for three reasons,
1), its expensive
2), it might not be needed if your current power supply is good enough.
3), power supplies drop quickly in price. If you are trying to by parts now for a computer a few months from now, you are better off to stick the money in the bank and get everything you want/need all at once. prices might have gone down.
I say that you should only upgrade if it is needed. hook up your computer and turn it on, if it turns on and it runs stable, then you don't need a new power supply. if it won't turn on, then get a new power supply.
Don't spend money needlessly.
Furthermore, you have yet to tell use the spechs of your power supply, besides that its a 400 watt, which means nothing. I could have a 350Watt power supply that is better then a 600Watt depending on how the voltage rails are divided.
Hope that helps.
