http://people.overclockers.ru/golos2/articles
It's a voltmodding guide of Sparkle 6600 AGP 128Mb by golos2.
The result is impressive - 614/803 and 8272 03'3DMarks. I also have such 6600, now it is running at 570/660 (no mods, just Titan CuD5Tb heatsink on GPU), and I tried to increase memory timings. It gave me 760MHz (+100MHz DDR accurately ), but perfomance stayed at the same level. Now I want to make heatsinks for memory chips and get 800MHz with this Vmod.
And if you have troubles with Russian language, I can translate some words.
First, comments on the main Vmod photo:
Green arrows - points of measurement (positive, Ground u can take where you like):
upper - memory
lower - GPU
Variable resistors:
For memory:
100kOhms, large black resistor
For GPU:
10kOhms, small metallic one.
Two other yellow arrows showing ground wires of the MOLEX connector.