Keep the HX620 unless you want to save a few quid. I had the option also and opted for the bigger PSU as you never know what you'll add to the system that might need it.
Rest of the system is looking great

, I'm certainly enjoying my gaming at the moment

. Not sure about the P35 boards with no experience with them but I doubt it should have any major problems but maybe best waiting on someone's advice that knows something about the board or have you seen some reviews at least.