IS there any recommendations on how much memory to run in a hub/cas (split role on one server). The operating system is 2008 r2 standard so is limited to 32Gb on that side but will Exchange cope ok with that amount of RAM or is it a case of too much RAM isn't always the best? Thanks

Take a look at: Understanding Client Access and Hub Transport Combined Role Configurations in Capacity Planning 8-16GB depending on your load should be enough in most cases.

With that much RAM I'd have 2 servers doing the HUB/CAS role. Just to give you an idea, for maiboxes with around 4K-6K up to 16GB is fine.Sukh