1) massively underclock the memory on your GPU cards to between 250-300 .. (you should be doing this anyway to save power) Unless taken to absolute extremes, this will not impact mining performance. It will, however, impact gaming performance if this is a dual-use rig.
2) If noise is a bigger concern than performance, underclock the cores on your GPU cards. This will also save power, and allow you to lower the voltage on the card, saving more power.
Underclocking is indeed the way to go IMO, if you don't want to use watercooling. That's what I did on my latest rig: I have 2 Radeon 5870. The bottom one underclocked at 750MHz (core) / 300 MHz (memory), the top one, which isn't as well ventilated, is currently at 550/300. I hope to increase those when room temperature gets lower...
You could also flash the graphic card BIOS to modify the fan speed / temperature curve ; unfortunately on my card this wasn't an option.
