Is it possible to use only the standard heatsink for a Duron 700 MHz. I want to switch the fan off.

Using a temperature probe on the heatsink fins I get a reading of no more than about 43 C (110 F) in normal operation.

This website says I can go up to 90 C .

http://www.pantherproducts.co.uk/Articl ... ures.shtml

Is it likely I will go as high as this without a standard fan and a standard heatsink?

If it's doing 43c with a fan then it doesn't sound good.
Sadly the duron chip is a bit of a power hog.
[url=http://users.erols.com/chare/elec.htm]Duron 700 = 34w -> 39w
Pentium 3 700 = 18.3[/url]

It won't work - it will just burn up.

I tried running a Duron 700mhz undervolted to 1.5v with a copper HS but fan disconnected and there was acrid burning within a minute, so I had to immediately disconnect everything. The HS was scaldingly hot to touch as well. I wouldn't recommend it.

I'm running a Duron 600 with an SLK-800A and a 80MM Panaflo L @ 5V and it's 'cold' (less than 40C under load).

I know you can't run a TBird 1.1G fanless even with a large heatsink. I would say no. Particularly with the stock heatsink, you need some kind of a fan. The best you could expect to get away with would be an 80mm "L" fan on that heatsink.

I guess this was without any heavy load?
If you got CPUBurn or even just normal programs that used 100% CPU I would expect that CPU to die within the hour.