I think you should be able to run mojoPortal within those limits as long as it is one IIS site per pool. Many budget web hosts have tight limits on memory available. Your mileage may vary if the sites have a large site with lots and lots of pages.
The thing with ASP.NET is that if more memory is available it will use more memory and it will run better if there is plenty of available memory.
I "think" when more memory is available .NET does not have to be as aggressive about garbage collection to free up memory.
Typically if you do not have enough memory available it will manifest as the application pool recycling constantly or you will see system.outofmemoryexception happening.
Hope it helps,