XP Activation and Dual Booting

Will I be able to activate a reinstall of XP after April, 2014?

The end of Windows XP support will not affect activation, but rather security updates and phone/online technical support.

A spokesperson confirmed that activations will still be required for retail installations of Windows XP post April 8. "Windows XP can still be installed and activated after end of support on April 8," the spokesperson noted.

In a posting on PC World, Microsoft publicly responded to affirm that Windows XP activation will be unaffected by the Windows XP support end date.

Source, Source, Source, Source

As for:

"..and dual boot another OEM copy of XP Professional (from a discarded machine)"

Legally you can't do that. OEM OSs die with the system they were sold with.