There is no real need to downgrade. If you have a 900w PSU and your PC is only drawing 400w, then the PSU only puts out 400w and the other 500w aren't used.
Having more power is always better, in case you get close to the limits, there is more overhead. I'd stick with what you have, why waste money.
Ah ok. Then I'll just keep it and use the extra money for the upgrades when I get to that.
Will just give the topic till tomorrow and see if there are any other answers!