How far can i undervolt?

I've got a P4 2.26Ghx 533FSB Northwood and to keep it nice and cool i want to undervolt it. I think i've currently got it on 1.375 volts from the stock 1.5 and the gigabyte CIA (Overclocks to about 2.4Ghz when it needs it) is enabled.

So basically how far should i be able to take this chip at 2.4Ghz? Got a thermalright XP-120 on it without a fan if that makes any difference. Current Temp (Running winamp/few IE windows) with room temp of about 20 degrees is 31 degrees acording to speedfan. Thats using 1 stock fan. Might as well keep it nice and cool, think its about 2 degrees cooler than with stock voltages