Have these components been purchased so far? Motherboard / Memory / GPU?
A quality 450watt will power it but most only have 1x pcie epress power connector
500watt from pc power and cooling will work well, or the 550watt from corsair, or the 750watt from pc Power and Cooling for $99 if you want to keep crossfire/sli possible.
http://www.newegg.com/Product/Product.aspx?Item=N82E16817703015
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139004
http://www.newegg.com/Product/Product.aspx?Item=N82E16817341011
I would really consider a different graphics card, Either a 4830, 4850, 4870 1gb, or gtx 260 core 216.
I would consider a P45 based board unless your certian your friend wants to keep SLI option open on the LGA 775 platform.
GA-EP45-UD3R if he wants to keep single nvidea or ati card
GA-EP45-UD3P if he wants to keep crossfire open if he selects a 4830/4850/4870 instead of 9800 gtx
http://www.newegg.com/Product/Product.aspx?Item=N82E16813128359
http://www.newegg.com/Product/Product.aspx?Item=N82E16813128358
A budget case that will work well with almost anything would be the CM RF-690
http://www.newegg.com/Product/Product.aspx?Item=N82E16811119137
The 300 is good aswell but it will barely hold a 4870, the 690 has more room for just a little more.
I would list your friends budget / resolution.