had the same problems with my 1.6A @ 2.1GHz. It would just blue screen and reboot, and strangely enough, wouldn't happen in games at all, it
would happen when i was just browsing the web or opening a folder randomly. I had tried everything from cpu voltage to putting all memory settings
down to safe settings. Then i got tired of dealing with drivers and possible conflicts with there new detonators so i did this:
Reformatted and installed WinXP Pro fresh, did the windows update online first (important to do it before any drivers), then install newest AGP patch (SIS 1.09f for me)
then install the newest detonators 28.32. And guess what, i've now been running for 3 days straight without one single blue screen, it's been rock solid.
Windows XP is just likes things to be done a certain way, and i learned the hard way how to do it right.
I had spent like a week trying to figure it out after i got my new Gf4Ti 4400 card. Don't waste anymore of your time, do what i did