Hi Passenger,
All information Veetguitar offered above is right. The main problems with your system will be CPU speed and RAM size if you really plan to go the MIR route.
But if you are "just" looking for an alternative to AltiVerb, then MIR is not the right choice, as it is meant to be much, much more than simply "another reverb". MIR suggests a completely new approach of mixing and arranging music in virtual spaces. This may or may not be what you're after.
Have you ever though about Vienna Suite? Part of this plugin bundle is the acclaimed Vienna Convolution Reverb. It is aimed at mixing audio the "conventional" way, like you would do other convolution plugins, too. -> [URL]http://vsl.co.at/en/211/497/1686/1673/115.htm[/URL]
There are literally hundreds of tailor made IR sets available for Vienna Convolution Reverb, offered by VSL as well as by third-party developers:
-> [URL]http://vsl.co.at/en/211/497/1686/313/1388.htm[/URL]
-> [URL]http://vsl.co.at/en/211/497/1686/311/181.htm [/URL]
-> [URL]http://vsl.co.at/en/211/497/1686/312/242.htm[/URL]
Kind regards,