I've had to use the documents I linked to in my post about Tuning the ThreadPool a number of times and it's a struggle to remember which elements in the machine.config each setting should be applied to. So I'm going to post all the details here for reference. N is the number of processors, including virtual processors in hyperthreaded CPUs.

Inside /configuration/system.web:

<processModel maxWorkerThreads="100" maxIoThreads="100" minWorkerThreads="50" ... />

Inside /configuration/system.web:

<httpRuntime minFreeThreads="88xN" minLocalRequestFreeThreads="76xN" ... />

inside /configuration/system.net:

<connectionManagement>
<add address="* or providerIPHere" maxconnection="12xN"/>
</connectionManagement>

This information is taken from this Microsoft Support Entry.

If you're lucky enough to be using .NET 2.0 or above you can simply set the autoConfig attribute of the processModel element to true (inside system.web).

<processModel autoConfig="true" />

However, note that these changes will only apply to web applications in this instance.