PSU definately may be an issue... I have the same system you described above but with a OCZ 700watt PSU. Its also not just the watts, if your PSU is older it may not have all the required connections... i dont remember what connections the GTX260 required but it may be different than the 480.
Also dont forget to consider your case size. Mine was a superclocked with upgraded/bigger cooling unit and it barely fit in my Raidmax Quantum case (which is one of hte bigger mid-towers on the market)