Even without underclocking, dropping cpu voltage on your current chip will have a big impact. (Be sure to test for stability, blah blah blah.)
How are you underclocking? I can't imagine a laptop bios lets you. What can you adjust (colts, base, multiplier, fan speed...)
Is the noise (or fan RPM if you have access to it) dropping at ide, or is it a constant speed?
You won't see much of a difference with a 770, I wound't bother with replacing the CPU.