Upgrading PSU and GPU in XPS 8300

Hi, I'm looking to upgrade the GPU in my XPS 8300 to a GTX 970 SC, which recommends at least 500W. In upgrading the GPU I then also need to upgrade my PSU I believe, but I can't seem to find one with the appropriate dimensions. I popped it open and measured the PSU currently at 135mm Long, 150mm Wide, and aprx 90mm Tall. Any suggestions for what I need to upgrade my PSU to would be greatly appreciated. Or, alternatively if I'm an incompetent scrub that doesn't understand he doesn't NEED to upgrade his PSU, please let me know that too. Thanks.