Hi War / PCTech
Many thanks for your responses and it's good to know that I won't be wasting my money on a CPU that may not meet the high hardware requirements for Longhorn. There were also some other requirements which I think you should know about (including those for laptop computers), so I am posting the URL for Paul's website and you can then read all of the details:
http://www.winsupersite.com/default.asp
Once there, click on the link for April 29th (2nd link down on the left) and then scroll down to the hardware requirements.
As I already have 512MB DDR400 RAM and a dedicated 256MB DDR Graphics Card, I think I may just get away with upgrading my desktop CPU as stated, but my new laptop only has an Intel 1.5Ghz mobile Celeron CPU and 512MB DDR RAM, so that with that 'spec', the laptop will default to the low-end Aero Express user interface.
Regards
Brave01Heart