XP Mode Memory Settings Change

http://blogs.msdn.com/b/ianm/archive/2009/09/20/changing-windows-7-s-xp-mode-memory-size.aspx

It's actually much simpler. Just click Start -> Programs -> Microsoft Virtual PC -> Virtual Machines.

Then right click the VM and click settings.

Note that if the machine is already hibernated, you'll need to shut it down which you can do by clicking (in the running VM) Start -> Windows Security, etc.