Sql server 2008 R2 - single instance VS multiple instance on the same box [on hold]

We have got one single dedicated Sql Server 2008 R2 box running multiple named instances for different databases for different country per say for UK, France, Germany etc. Overall we have six databases running under their own individual instances in single box. And the box is having very limited memory (as always) 64 GB.

Facts:

1. All applications (multi-tenant) connecting database with the separate credentials using ADO.NET


2. All databases having their own SSIS packages, SPS etc. I mean, there is no common (shared) data across databases


3. There is no stringent security requirement


4. All instances are having their own disk. I mean, as of I assume, they don't compete for the same controller


5. All the instances are configured with min/max memory settings.(Not sure if it can be done for multiple databases within same instance)

We have been facing performance issues. Apart from exploring other options, one of the option we want to consider is to have all the databases under one instances. Is it a good practical idea? What could be pros/cons? What happens if one of the database is hammered, will it effect other databases in the same instance?

Please share your thoughts, experiences, links, blog, white paper... anything is highly appreciated.