Well i'm not sure why a well know PC manufaturing company would deliver you pre-built pc with an inadequate PSU.
If you really are concerned then I would look up some reviews on this model. Like TrustedReviews?
Plus if you want you could buy a psu and save it incase you need it when the PC comes, and if you dont then you could re-sell it on E-Bay or keep it as an additional PSU for yourself
Heres a link by the way to the product on the ASUS International website:
Here you could find additional genuine specs..#
Im just noticing that it has a 6 Core Phenom :/
Thats odd for a 300w.....or is it?