The thing is, most branded computers such as HP, Acer, Dell, etc. Go cheap wherever they can, and the PSU is something where they really get cheap.
The PSU may have a rating of 460W, but can it really output 460W full load for a long time? You can bet it won't. That's why 80+ Bronze PSU's should be bought when pairing high tech parts, to be sure that it's a reliable PSU.
Oh so would you recommend the Asus GTX 750 Ti 2gb GDDR5 or actully a GTX 660 2gb GDDR5?