If the VM decides to use all the memory, then it will increase the time between the garbage collections (20 sec instead of 4), but it will also increase the garbage collection time by 4, so every 20 seconds there will be one massive lag spike. This would need a PC with at least 8 GB RAM ( as the real memory usage is almost double the memory visible in Java). Let's give Minecraft 4 GB of RAM to play with.
