Is an amp an absolute must with this if you already have a motherboard with good amping capabilities (AsRock's PuritySound 2- supposed to drive up to 600 ohm headphones)?
SlimceaAn amp isn't an absolute must with these headphones at all. If you aren't using an amp you won't be able to get the best sound out of them (that is in general, not about your card specifically. Your card might be fine but I don't know) but most devices can still drive them to sufficient volume. I would recommend waiting until you get the headphones and then deciding if you think you need an amp.
SlimceaIf the sound on your computer is an integrated chip on the motherboard, I would find a way to amp them. More power and better quality will really open them up.