We are looking to improve our infrastructure, which is currently a single machin

On one hand you are have dedicated server for each component, which give you more predictability of performance but you are also increasing complexity of your infrastructure and configuration. The only reason why I would move the components into separate machines is in a situation where the current CPU count is overwhelmed by the load and is consistently over 80% in use. Is that what you are experiencing?

CPU usage averaging very low average 1-2%. However RAM usage is high - we have a single server with 64GB RAM currently running about 50% utilisation . Our aim is we want to build a reliable and sustainable environment which we can increase process numbers (within reason) without experiencing any detrition in performance or worrying about memory usage. Any suggestions to what the optimal server configuration might be? At this moment in time we can only envisage an on-premise environment.

Joe, it will be very hard to give you any recommendations without any data points in terms of your process volume, durations, reporting needs, etc. I would recommend doing a Health Check on your existing system, which will assess the current status of your infrastructure and recommendations for future growth.

Just to clarify, Appian PS offers a Health Check package as part of its services offering (www.appian.com/.../overview.jsp). Please contact your account exec to set it up.

We run seperate servers for Engines, JBoss and Apache but I can't give any confirmation that there are performance benefits. Whilst maintaining multiple servers adds to complexity it also can make maintenance easier and it can help to identify bottlenecks. Definitely agree that you should get PS to check out your load details and get a recommendation.