If you are going to go that route and not build a pc yourself save yourself some dough.
What other psu choices are offered?
A 500W psu will power your present specs.
I'd buy the lowest power good quality unit offered.
If everything costs more then the default unit = stick with it.
It cheaper and quite easy to replace a psu later on if you decide to SLI.
Just had a look a generic 500W is the default unit.
Buy something decent later on from NCIX or Newegg.