You should have no problem with putting a 500W Power Supply(PSU) into your system assuming that your computer already has a standard sized ATX power supply. Most 500W PSU's are the same size as a 300W PSU.
You should have no problem fitting the GTX 760 in to your case since most cases provide around 10" or more of clearance for add-in cards. The best way to tell is to measure the clearance inside your case with a ruler directly above the PCIe slot you plan to put the card into.
However, it is very important that you make sure that the PSU you buy has the correct number and type of PCIe Auxiliary power connectors. Most GTX 760 cards require two 6 pin PCIe Auxiliary power cables and a few require one 8 pin and one 6 pin. I would...