The computer that your getting has a 480w psu in it and the video card that you want requires a 450w power supply so the psu that comes with that computer is good enough for that video card.
So your taking out the video card that comes with the computer?
Yeah, the 480 W PSU in there isn't meant for a system like that, i am actually surprised it will run that system at all. If you go into the pictures, pick the one that looks at the side with the side panel off, you can zoom in on the PSU and read the load label.
3.3 V 28 A, 5 V 36 A, 12 V 16 A
That is only 192 W of 12 V capacity that is trying to feed a system with a CPU that eats up to 95 W of 12 V power, and the graphics card you picked out needs around 130 W at peak, plus the motherboard and drives need some 12 V power, it is definitely a good idea to replace that PSU, and a very good idea if you intend to put that GPU in there.
The Corsair CX 500 that paddy linked to would do much better.
The Antec earthwatts 500w does not come with a power cord so you either have to buy one or use the one from your old computer if you have one.
SeaSonic is one of the top brands for making power supplies and has the top reputation of dependable and efficient power supplies.