The heatsinks used in PSUs are generally soldered to the PCB via little metal tabs and current usually flows through them, ie they are live with voltage and act as part of the circuit. What you have to do is cut the tabs which takes the heatsinks out of the circuit and them use wire to replace them in part of the circuit.

It might look like this when done (the wires are where the metal tabs were):

The second part can be done various ways eg. make heatinks that replace the existing ones and attach them onto a bigger heatsink (thats what I did below) or extend the transistors with wires and attach them directly to the big heatsink.

You can extend them like this:

Here are 3 other articles that have used the extended the transistors method:

www.silentpcreview.com

www.zerofanzone.co.uk

www.digital-explosion.co.uk

I tried this method on three PSUs all with no luck. The PSU would whine or hiss after adding the wires. I tried making the wires really short and I checked all my connections repeatidly. I asked a PSU repair guy why and he said that it was the extra resistance the wires made in the circuit that caused the noise. He aslo said that it may seem like a really tiny difference but it was because the opperate at a very high frequency. I wanted a REALLY silent PSU so I was particularly fussy about the noise it produced so if you did the same you might not notice or care. Maybe I just had bad luck and picked the wrong PSUs, I dont know but it worked like this anyway:

I was lucky and got a 150 watt Micro ATX PSU in which the heatsinks were grounded:

I had to remove a capacitor to unscrew the transistors from one the heatsinks:

The other I had to completely remove transistors and all because there was no way to get a screw driver in there to remove the screws on the transistors:

I then made similar heatsinks out of plain 5mm aluminium. I did this because the original ones were going to be very difficult to bolt to a big heatsink.

And then I put the PSU all back together:

Then I cut the space where the CDROM would have gone in my micro ATX case to make room for the big heatsink:

Then I bolted it onto the chassis:

Then I made 90° aluminium parts that would bolt onto the big heatsink and the little ones on the PSU:

And attached them to the PSU like this:

And I drilled some holes for passive ventillation:

And I made a plate for the back to hold the IEC power socket and switch:

I use a socket 7 Pentium 266 (Tullimook core) at 100x3 Mhz at 1.9V, this CPU was from laptop and runs very cool.