There is no harm in getting a 550w when you need 430 or so, except your initial
cost will be higher. The total watts will not affect your electricity bills, as
the PSU will pull only from the wall the power needed by your PC
components. A more efficient PSU will use less power from the
wall than a less efficient PSU.
If you have high electr rates, you should consider a gold,silver,
or bronze rated unit.