The best and pretty well known tool for overclocking graphic cards is Powerstrip from EnTech, Taiwan. In the 'Advanced Options, About the Powerstrip'-menu there is a submenu 'Performance', which lets you switch to fast memory settings and which provides sliders for adjusting the memory and chip clock.
To make sure that a card can really cope with the new speed, I was running my 24 minute Quake2 'tbone.dm2' demo, which is based on my favorite map 'sewer64'. This demo is giving the whole system a really hard time and will make any unstable card crash. Sometimes you only see triangle drawing errors, which is a safe sign that you overclocked too much. Here are the results that I found:
Swipe to scroll horizontally
Product
Highest Chip Clock Setting
Highest Memory Clock Setting
Asus V3400TNT
120 MHz
125 MHz
Creative Graphics Blaster RIVA TNT
105 MHz
115 MHz
Diamond Viper V550
105 MHz
115 MHz
Elsa ERAZOR II
100 MHz
120 MHz
Hercules Dynamite TNT
115 MHz
125 MHz
STB Velocity 4400
100 MHz
120 MHz
The best memory seems to be used by Asus and Hercules and those two would also allow the highest chip clocks due to their active cooling devices. Diamond and Creative let the chip clock go up to 105 MHz, but the memory can't cope with more than 115 MHz.
