I'm just going to 2nd what max said, people always overlook the PSU, but it's very important. When I'm building a PC and someone gives me a budget, I look at what they want to achieve with the PC, then pick a PSU first thinking about what they want to have in the PC. You don't need to go all out, but a good quality PSU with enough wattage to run what you want should always be your first decision. So get a new PSU if you plan on upgrading your PC.
That GPU asks for a 500w psu min. Get something decent, search this site or the web for a tier list of decent to good brands.