|" .....it's when you begin upgrading it with, for example, more drives or a more powerful video card, that you need to look at upgrading the PSU so it can cope with the extra demand."|
Addtional hard drives and optical drives draw very little power.
If your friend thinks they will NEVER need to install a video card in a slot on the mboard, it doesn't matter which model he or she gets.
However, if your friend ever installs a video card in a slot....
Your power supply must have at least the minumum capacity required to support a system with the graphics card you are using installed, or the max graphics card you might install in the future.
(Onboard video - video built into the mboard - IS NOT A CARD!)
You can go to the video card maker's web site and look up the specs for the model - often under system requirements - the minimum PS wattage, and, more important, the minimum amperage the PS must supply at 12v is stated. If you don't find that, any card with the same video chipset including any letters after the model number has very similar minimum PS requirements.
You MIGHT be able to get away with a power supply capacity 50 watts less than the minimum stated for the video chipset, but NOT likely more than that.
The 620s has an oddball proprietary sized power supply.
They cost more to buy for the same wattage capacity, very few video cards you can install have a video chipset that will be okay with a 250 watt PS on the system without frying the power supply eventually, there may or may not be replacement power supplies that will physically fit in the same space as the original one that have a higher wattage capacity, and if there are, the wattage choices you can get are limited and meagre.
The 620s may be able to use only low profile video cards - that limits your choices .
The 620 probably has a regular sized power supply, and if so, the choices of replacement power supply are unilmited.
The 620 probably uses regular height cards - lots of video card choices .
There are probably more avalable wiring connectors coming from the 620's power supply.
You can probably physically install more drives in the 620's case.