JBoss will have benefit from a multicore system.
But all the parameters depend on the application.
In general it will be good to have a fast IO to disc to write the logfiles and store some necessary data (tmp or data).
But if you have not a big amount of data stored on a disc increasing the speed of IO might have no benefit for the application.
The amount of memory is the same, if you have an application with a small footprint, i.e. fast actions with only small data, a big heap might not good as it takes long to clean and adjust it and a smaller one will have less pauses.