Multi-tenancy web application hardware requirement

 Multi-tenancy web application hardware requirement

Dear friends,
I want to create a Multi tenancy web application with following requirements
1. Cloud based service
2. 5000 -10 000 users
3. share database, web interface, OS, physical marchine between tenants

I want to ask about the requirement of the Hardware configuration for this system.
Your answers are great appriciate!
We use Rackspace:

* 2 app servers, their 8 GB plan, load balanced
* 1 DB server, also the 8 GB plan
* We are about to add a second DB server, mirrored, as a hot spare

This is handling a 400,000 SU application, 40,000 users, 200 tenants. 15% CPU usage on the app servers during PEAK load times.

If you're doing a SASS app (which I'm assuming you are, since you ask about multitenancy), the load balanced app servers are VERY important since they let you do maintenance, troubleshooting, etc.