On the Amd radeon website it indicates that you will need a 500W psu or greater to run a hd 6850 - personally I think you would be fine with a 450W (it only uses 127W on load). It is definitely worth getting a psu with a higher wattage just in case you upgrade in the near future - you don't want to have to buy another psu. Hope this helps and good luck with the build!
1: id get a cheaper board like the asrock h61 dgs. you wont need much on this budget build
2:with the savings you can get yourself a 6870. its 144.99 after MIR at newegg (xfx branded)
3:as for the case, id very least get a fractal core 1000 case. i really dont believe in poorly built cases
4:id get this monitor but it is double the price. very good unit http://www.newegg.com/Product/Product.aspx?Item=N82E168...