What OS are you running? If windows then they will fit easily, because you can go very low on wattage with right WattTool profile. Important information will be your current energy consumption and if your 470 cards are tweaked in any way.
yes, If he is using windows, no need for a custom ROM, just use Crimson and set a decent undervolt for the core and memory.
Dude .. it you dont know stuff. please dont advice. Without rom modification its impossible to get rates about 25MH.. With rom mod I assure you can go above 29MH with using 30% less power than unmodded.