Your idea, while workable, is not valid. You will actually slow down your system. ReadyBoost takes advantage of the flash memory speed, which an HDD does not have in comparison. Also, ReadyBoost involves simultaneous read/writes over two subsystems. If you use an HDD, these actions will require additional wait states.
You could invest in a PCMCIA USB port card and a USB wifi device. You will then be able to do both, wifi and ReadyBoost.
Your machine would likely benefit from ReadyBoost.