I have been evaluating switching PCs from Windows 7 Pro to Windows 8 Pro on a VMware client for some time using an MSDN license key.

I've resolved all my questions and I'm now considering buying a Windows 8 PC.

My question is:
Can I use the Windows 8 pro licence key that I was using on the VM client to upgrade a new PC purchased with Windows 8 Home to Windows 8 Pro?

As long as you decommission the Virtual Machine, you can move it the license to the new PC.
Press Windows key + X
Click System
Under Windows Edition, click the link:
Get more features with a new edition of Windows
Click the 'I already have a product key link'
Enter your product key and you will be upgraded to Pro.
You will need to reactivate by telephone since the license was previously in use:

Phone activation:
1. Press Windows key + X then click Run, then type: slui.exe 4
2. Next press the 'ENTER' key
3. Select your 'Country' from the list.
4. Choose the 'Phone Activation' option.
5. Stay on the phone (do not select/press any options) and wait for a person to help you with activation.
6. Explain your problem clearly to the support person.