However, I also then realized that not only minimum wattage, I have to consider the Amperage of the PSU so...
My GPU says it requires a minimum of a 400 Watt power supply. Not sure if this is the number I should be using, but I don't have any other number so I'll take 400 -> 400/12 = some 33A. I'll add some 15A for my other stuff, and 5A for a safety margin, so I should be looking at some 55A for a PSU, correct? Or did I do something wrong...
The Antec PSU says it can give a total of 336W -> so that means its total amperage for its 12V rails are 28A, which seems to be nowhere near what I require so...
Once you get past the basic requirements for most of the components of a typical pc like yours,( perhaps 100W) it is the graphics configuration that determines the power you need.
Virtually all of that power comes from the +12v part, and it is the total +12v amperage that counts.
Cheap psu's will claim high wattage, but not deliver the watts on the +12v rails where it is needed.
A simple way to size a psu is to buy one that has sufficient 6 or 8 pin pci-e connectors for your graphics card.
The Antec earthwatts 380w unit will be fine for your GTX650.
If you want, it would not be wrong to overprovision the psu a bit.
I might look at a 600-650W unit.
A psu will consume only the power demanded of it, regardless of it's maximum capability.
A psu will operate more efficiently and quietly in the middle third of it's range.
A stronger psu will allow you to upgrade to a very strong graphics card such as the GTX680.
Ah many thanks to everyone for their replies. I see where I went wrong in the Amperage calculation, though that does make me wonder...
How do I figure out the wattage that my GPU consumes? Is it on a chart somewhere or is there some other way of figuring things out?
A pci-e motherboard slot will power many graphics cards by itself.
It can consume up to 75w(or 75w/12v) or 6.25W @12v.
Each 6 pin pci-e power connector that your card needs can consume up to another 75w.
Each 8 pin pci-e power connector can deliver up to 150W
If you allow for all of these "up to's" you will have an idea of the maximum power draw of your card.