iOS4 - fast context switching

Question

Hi All,

When application enters in background running state, how much dirty memory usages is good to go. In apple video it's mentioned that the dirty memory should be reduced as much as we can.

But in my App, I am using navigation controller to push and pop views. After moving from about 20 different pages, the dirty memory usages reaches 30 MB or so.

Also on "dismissModalViewControllerAnimated" and "popViewControllerAnimated", dealloc is not called.

I have two doubts:

1. With how much dirty memory is acceptable to go live?
2. What is the alternate of navigation controller to support back button?

Thanks in advance.