Thanks so much for your help everyone, as you can probably tell, I'm rather the enthusiastic amateur when it comes to power supplies!
I remembered that one of my friends had an old pc in his garage, so I bribed him with beer and got a new power supply.
This is a big deal, because this supply has got 3v, which the other one hadn't, and that will be really useful to me.
One thing about this one that I don't understand is that it has two +12v rails.
One is 10A and one is 15A.
The 15A rail has only two wires connected to it (they are yellow with a black stripe as opposed to solid yellow).
This board is different, and doesn't have nice big holes to put chunky wire through like the other one did, so I'm stuck with bundling the 18 gauge wires together.
It seems odd that the 15A +12v rail only has two wires, whereas the 10A one has 7 or eight.
I only need one +12v output, so should I go with the more powerful (and thus more useful) 15A one (which only has two wires) or the 10A one with several wires?