I originally bought the PSU for my old computer build but it was too big :S. Do I need to buy a new PSU? I'm open to the idea anyways since I'm upgrading my CPU and graphics card so I believe I need more watts anyways. My cpu takes 125W and my graphics card says minimum 400W supply. Recommendations would be appreciated.
Your case takes a standard ATX PSU and the one you link is not a standard ATX size so you are right that it will not fit. From the limited info you give regarding power requirements, any good manufacturer 450Watt PSU will satisfy your needs as long as it is 80+ or better. BTW you do not add the 400W and 125W together, the GPU manufacturer is allowing for the platform needs too.