Horizontal scaling

Today we finished conversion of a medium-load (~1.2k requests for dynamic content per second) application (frontend, C#+ASP.NET Web Forms+ASP.NET Web Pages) to work on two separate machines. While moving databases around and adding memcached memory was relatively easy, splitting the application that contained state (lots of internal caches) proved a bit hard, even we already have a plan for proper user affinity (cookies+ip hashing via haproxy). PHP users, for example, do not have the luxury of large-scale persistent internal state, so they do not plan for it and use external devices like memcached, message queues, etc. Our application was written in ASP.NET and used about 70 internal caches when I started working on it. Well, now it perfectly runs on two machines behind NGINX+haproxy, and where's two, there's three and more =)