Using VM's to run WebLogic with Coherence

Hi,

Does any have any experience or suggestions regarding using Coherence with WebLogic?  I'm interested in architecting an environment in which we use Linux running on VM's, so if we have capacity problems we can add another node quickly (scale).  Also, I would like to be able to use jRockit JVM rather then the native O/S version we are using now.  

One of the big issues we have is memory use, we have to spend a lot of time tuning the JVM heapsize, running GC options, etc.  I'm talking about environments running 20+ webapps spanned over many clustered JVM.  Of course we have tools to make things easier, like WLST,wlnav but another pain is the WLDF console extension doesn't really work well with current JVM.