If your new fans give you a fair amount of airflow over the mainboard, I'd still recommend looking at zalman - if the CPU's not a really hot one you can get a copper heatsink that should cool things real quietly. Or, you could try getting a larger CPU fan, like an 80 or 120mm if there's space - you can get wide, flat heatsinks that'll let you fit a big fan. The idea there is that a large fan can shift the same amount of air as a smaller one, while spinning slower so it produces less heat.