Here is the syntax to capture a memory dump using adplus (part of the debugging tools).   FYI if you need to capture a ultra large w3wp process, you will need to increase the ping timeout inside IIS so the dump doesn’t fail.  IIS will detect a ping failure, and recycle before the memory dump has finished writing to disk.  I’ve used this since IIS DebugDiag doesn’t work on 64 bit of windows server 2008.

adplus -hang -p <PID> -o path to file 

http://support.microsoft.com/kb/286350