Does anyone know what are the system requirements for the newest version of MIRC?

http://www.mirc.com/faq.html#section3

That basically sums it up. I think you can safely assume any computer less than 10 years old can run it.

Thank you for that, but more importantly how much RAM should it use?

Under normal conditions mIRC will not take up more than 20MB RAM at any given time. When using certain scripts or with huge (non-default) window buffer sizes this can increase.

Thank you for all your help.