Datastax recommends to have 8GB limit on JVM's memory “In most cases, the capability of Java 6 to gracefully handle garbage collection above 8GB quickly diminishes.”
Is it the still the case with Java 7?
It is claimed that garbage collection was significantly improved there:
"One of the first improvements that Java 7 brought in was G1 garbage collector. I quote: “The G1 collector is a server-style garbage collector, targeted for multi-processor machines with large memories. It meets garbage collection (GC) pause time goals with high probability, while achieving high throughput. Whole-heap operations, such as global marking, are performed concurrently with the application threads. This prevents interruptions proportional to heap or live-data size.”