Since you asked for observations of Task Manager memory usage values after 5-10 minutes of IDEA running, I thought I'd start a thread where everyone can report their values.
For me, in the Task Manager, VM Size is 253MB, Mem Usage is 257MB. This is with the default VM settings from a fresh zip install. The IDEA memory indicator says 89M of 124M. If I click the garbage collect button, that drops to 63M of 124M.
To be honest, I can't recall what the VM SIze and Mem Usage values were from previous builds.