Having more than one pump is not necessary, it will make no difference to your temps. A decent pump will give you more than enough head pressure with that setup so you could use a storm block for example. As another poster stated you are better getting a larger rad as I take it you are using a single.
To answer your initial question it does not matter what volts your pumps are, however, it is pointless so is irrelevant.
I take it you will be using a decent pump so the size of your rad is what counts so that you can dissipate as much heat as possible a clearer understanding of your rig and what parts you are looking at would help.
Reserators suck.