micro-atx or mini-atx psu should fit in that casing.. mostly psu that has M-suffix (Corsair psu):
http://www.newegg.com/Product/Product.aspx?Item=N82E16817171092
http://www.newegg.com/Product/Product.aspx?Item=N82E16817151096
You don't have to worry about 750w requirement of R9-290, the card actual power won't just go beyond 400 watts draw from psu... Here are few reviews and test results (total system power / 95w without graphics card)... The PSU with 500w to 600w is sufficient to handle your card and your system if you are not into overclocking..
You may have to worry about your card will fit inside your case
http://www.hardocp.com/article/2013/11/04/amd_radeon_r9_290_video_card_review/8#.VE33FiKUfVo...