I know this is not the right section for this question, but most underclocking is geared towards laptops and I figured I would get the best responses here rather then some place else. I hope that is ok.

I have an AMD opteron 165 in my linux machine right now. This CPU is clocked at 1.8ghz and consumes 110 watts all by itself, I know there is massive underclocking room for this processor, and even at it stock speeds it remains mostly idle in linux. I don't know the exact model motherboard I have, but I do know it has an Nvidia chipset, PCIE, and supports Cool&quite. According to PHC website the tool supports K8 processors that also support coolnqite but I think my processor is K6. Will this tool work? Does it work on mobile semprons from the same period? Is there another software tool I can use to undervolt, preferably ones that will allow me to make dynamic custom profiles based on CPU load? Thank you for any help.

Yes this is very possible if your cpu supports it and even sometimes it will even when others say it wont. I used to do this back a while ago on one of my laptops but it never really did give any sort of realistic battery increase. You can do this via kernel tools that by now im sure are included in the kernel (back in the day you had to patch them in). If memory serves me you had to make changes to a file in /proc that would set a lower voltage and multiplier for the cpu. From what I remember you could only go so far down before you got instability. Also remember that some of the AMD cpus are locked and you cannot underclock them. Another reason for me using the phenom black edition since its unlocked.

I know my opteron can go down in multiplier. I am primarily concerned with undervolting it at stock speeds, I know chips very similar to this can go to much higher speeds at stock volts I am hoping I can do the same speed at much lower voltage. How exactly does this editing of the proc file work? Is there any guides online I can follow for slackware or something similar.

Actually PHC is what I am refering to. You can rebuild your kernel with PHC patches and do the undervolting on the cpu. There is a nice tutorial on it here