Power supply requirement

What graphics card will you be using? Or will you use the integrated graphics?

A 420W psu is more than enough, for that setup, but I'd suggest sticking with a good brand of PSUs, such as corsair or antec.

I'll use the graphics in the on-board 780G. I used to play video, sometime HD up to 1024x???, on my desktop. Will this increase the temperature of the CPU and the 780G chipset much? The CPU will come with a heatsink and a cooling fan. It seems the main board does not provide a fan for the 780G. Is it necessary to install one?

I'll never understand why people try to use a power supply that's just barely big enough for their current build. My philosophy is to buy the largest quality supply I can afford. IMHO, it's the cheapest insurance policy you can buy for your new computer plus the supply will probably be usable for at least one more (if not more) future build. Power supply failure rates are related to how hard they are working. In other words, how much of their capabilities are in use typically. The harder they work the shorter the failure interval, all other things being equal. Why take the chance? There are so many other things than can happen that are beyond this kind of control that the last thing I want to worry about is whether my power supply is too small.