I'm planning on launching a web application soon, and have decided on using Rackspace's Cloud offering with Debian. I'm not expecting that much traffic to start, but would rather get the architecture correct now even with the small VPSs. The thing I'm not quite sure about is how many VPSs I should get. At a minimum, I know I'll want three VPSs:

• Two Apache webservers
• One server for MySQL

I'd also like:

• Nginx load balancer
• MySQL replication
• memcached

I'm not sure where those last three processes should be running. Can the load balancer run on the same machine as the MySQL slave, or should they each run on their own machine? Does memcached run alongside the webservers or on different machines?