I don't see myself needing 8GB anytime soon.500W is fine
the i3 runs pretty cool with the stock heatsink (20-30 celcius with a good case)
Try getting 8GB RAM if you can, adds about +£10 to your build
Remember good peripherals are important (mouse, keyboard, monitor, headset) so don't skimp down on them.
Other than that your build seems pretty good
If you had the budget for it get an i5 and a SSD drive as it will really improve performance and your computer experiance.