For gaming you will get no benefit from the upgrade if you don't upgrade the graphics card.
For general use, the benefit would be just about noticeable, but hardly worth it.
For CPU intensive apps like video editing, video converting and photo editing, well then that's a different matter.
I assume the 420 in your psu is the power. This should be ok, but you should get a 600 watt one when you get a decent graphics card.