What do u think about using a Thermalright AX7
as a passive cooler on an underclocked Athlon XP mobile CPU???

http://www.thermalright.com/a_page/main_product_ax7.htm
My plan is to remove the fan from this cooler...

Why do you want passive cooling? Are you gonna be working in somewhere where you need no noise at all? I think you'll find that it'll be fine when your computer is idle but start anything cpu intensive and the temp will shoot up. I once tried running a Zalman CNPS6000CU without a fan with my old p4 1.9. It was fine but the idle temp was around 45 and as soon as i started any vaguely hardcore process the temp shot over 60C. With a p4 as it gets hot it will slow down to compensate, but an amd will just melt.
I'm using a Zalman CNPS7000A-CU and when i turn it down to the slowest speed, i can still do most work on it (UT2004 at full res is still a bit much) and at night, the hard drives are louder than the fan.
You could try a specific passive cooler like the HeatlaneZen NCU-1000 but i think that is for socket 478 only.
Hope that helps
Rizla