when the video card says it needs a minimum 500W it's saying that the whole system could need a 500W PSU. video card manufacturers 'overestimate' power supply needs because they have to take into account cheap power supplies that can't actually support their advertised 500W output. depending on the quality of your power supply you could probably run your system on anything from a 380W (if you got a high quality one) to a 600+ supply that is cheaply made and cannot actually produce the rated watts that it says it can.
can you tell us what power supply you have? they often have stickers on the side of them saying that stats. can you also tell us the CPU and graphics card you are using/planning to use?