Well the first point depends on what platform and how the kernel memory allocator is configured. It's possible to overcommit on Linux for example which is acceptable for short windows. NT will most likely tell you to go away.
Now that last point is the important one and is actually what I do which I was hoping to hear somewhere :-)
You can reserve more address space than physical memory just fine on NT. Committing beyond the amount of physical memory available will mean paging at some point, should you try and use all of it.
Now that last point is the important one and is actually what I do which I was hoping to hear somewhere :-)