What Power Supply will work with GeForce GTX and my AMD 64 x2?

Could you help me with advice?
I bought the following PC.

http://www.buy.com/retail/product. [...] 27&loc=101

Now I want to add a GeForce GTX card

http://www.newegg.com/Product/Prod [...] 6814143080

I do realize that I will need a larger power supply than the 250 W my new PC comes with.
It says 475 W in the spec for the graphic card. When I research power supplies it gives me
models starting on from 19.95 Dollars with 480 W already for that price.

http://deals2buy.pgpartner.com/sea [...] ort/popup1[]=170:616/sortby=priceA

How do I know which ones will work fine with my computer for the lowest price? When you look
at the graphic card link they recommend one with 800 W but it cost over 100 Dollar extra.

Would really appreciate your advise.