android内部和内存/处理器限制？

I'm considering creating a screen reader for Android. My questions are as follows. One how much of the internals of Android are exposed through the Java API? I'm looking for things such as system wide notification when text is displayed, notification of a new application being launched, etc. I'd rather not have to modify the kernel source and only allow my app to run on a developer phone. Two, how much processor and memory are left over on the G1 assuming no applications running? Three assuming average use of applications on the phone how much processor and memory are left? I'm considering creating a screen reader for An