well, since each card and chip and ram will have different overclocking limit, then i dont think that how far it overclocks is a good test of the heatsink...
and for monitoring the temps, you could get a thermal probe, and mount it on the bottom side of the card....i think that might give you a general idea of the temp, sure it isnt the surface...but close..
another thing you could try, is put it into the base of the heatsink...if you feel confident in drilling a little hole in your heatsink (with the hs removed from the card of course!)
You can use a digital thermometer called "CompuNurse" It's a small LCD screen and has a thermal sensor on the end of a wire, which you can fasten to the video card's heatsink, or anything else you'd like to monitor. I'm sure there's a few places you can get this item from, but i know that www.1coolpc.com sells it. Check it out at: http://1coolpc.com/tempcheck.htm
There's a picture at: http://1coolpc.com/tempcheck1.htm
It doesn't show fahrenheit, only celsius.
but to get accurate temp readings, you have to get the thermal probe as close to the gpu as possible.
if the fan on the hs is blowing on the thermal probe, then it is going to give odd readings.
leave the hsf mounted, and put the thermal probe under the hsf, on the side of the gpu..
dont wedge it under it, or it will mess up the cooling.
another thing you could try,
put a groove in the bottom of the hsf that is big enough for the thermal probe.
then put some arctic silver epoxy on it, and lap it down so that it is flush with the hsf base.
i mean flush!!!
and make sure that the thermal probe is close to the surface.
that will give you some accurate readings!
oh yeah, and let the as epoxy dry completely before lapping.
Or just buy the same card i have: a leadtek winfast ge3 TI500, it is capable of Hardware Monitorung. I haven't installed it yet, but i think that it will tell you the temp of the chip. I know that there are other boards out there that can do HW monitoring too, but this one is the only one i can think of right now.
since thats a really expensive solution, i'd also suggest to use some kind of thermometer
Asus cards come bundled with Smartdoctor, which tells you the temp, speed of the fan and can also slow the GPU down to keep it cool. And it can also slow your CPU down for the same effect. I used the monitoring function when running win98 but xp doesnt seem to support it.
-- There are no answers to find in the bottom of a glass. But you can have fun looking for them.