What are all of your system specs? That PSU has enough power to run almost any single-GPU system. It is compatible with any motherboard that requires either a 4-pin or 8-pin power connector for the CPU (i.e. it is compatible with ATV12 and EPS12V motherboards).
What PSU are you replacing with the Antec Neopower 550W?
Th CR and Asus links worked. One of things that I'm not sure about is the GPU that will be in your specific PC...Asus gives multiple options and CR's had a GTS 520. What video card will your PC have in it? If your PC is coming pre-assembled and tested, then why are you upgrading the PSU? Are you changing other components after you get the PC?
I am putting my radeon 6670, which is better than the gts 520, in it. The card requirese a 400w power supply. Plus i plan to add on the computer, as a save money up, like a higher end graphics card. I bought tthat specific pc cause reviews of it said it was a good computer to upgrade. And the first step in that would be upgrading the power suppy.