Well, I usually figure out what my system will use at peak consumption, then get a power supply that delivers a bit above that. I've been told by others that a power supply that is too powerful for the system may not run very efficiently, as power supplies are built to work best within a specific power range.
I don't actually know if that's true, or how much of a problem that would be if it was. But I have had success buying the best quality power supply I can get, that delivers a bit more than I'll actually ever need. I usually don't spend extra money on a unit that is way overpowered for my system.
That's what I've done in the past, and its worked out ok for me. YMMV.
In regards to video cards, I'm honestly not sure. I imagine if you are working with video files, or something, it might be a potentially big factor. I have a fairly decent card that used to be in my gaming machine, and I use it because it has three outputs, and I like to work with three monitors. But beyond that, I am really not sure what the best approach would be.