Actually I don't think you need a 1250. I have everything (cpu, vga, hdd, nb) watercooled, except the psu, and I use a 1046, which has half the capacity of the 1048, but I have 8mm hoses and a very, very large radiator. I estimate a 1048 is more than enough. More flow doesn't equal better cooling, although some will argue with that and say that to get that last degree, you need a high flowrate, but you're going for silence, so I think you're better of with a 1048. If you insist on a 1250, consider that these Eheim pumps don't generate much noise themselves, but they vibrate a little. If this vibration is transmitted to your case, your case get's very noisy. If you suspend the pump or put it on foam inside your case, you won't hear much.
Btw, how do you think 15 mm hose will cool your computer? Or do you mean copper pipes? Plastic hoses don't cool to well