It seems that even if you enable your modern Dell-ish computer in hardware (BIOS) to accept Magic Packets/Wake-On-LAN, if your operating system sleeps/suspends your computer you will not necessarily be able to remotely wake it.
A small sample of 2-4 year old Dell Optiplexes with WinXP and Vista installed suggests that "most" network drivers do not enable WOL from sleep/suspend by default.
I had hoped that a group policy template would allow us to rapidly turn this feature on, but alas it doesn’t exist. And the more I think about it, the more obvious it is: you can have multiple NICs on a machine, each one of which can be enabled to wake the system. So the registry settings may be different in each hardware configuration.
One needs to configure the NIC (Properties on the physical NIC, Configure button on the main Networking tab – then on the resulting window – the power management tab) as shown here:
The "only allow management stations to wake the computer" is optional; when checked, your machine will not "accidentally" awaken when the network card is checked by the OS or other Windows "housekeeping" tasks.