Well, you've learned something: don't ask technical questions on a porn board.
With PAE, your system can support all 4GB, but unless you obtained it installed that way, you may have less memory available. With PAE and a 64 bit capable CPU, it can map memory around, which is basically a more technical rehash of EMS page frames.
3GB is almost always safe. It's cheap enough, so populate that bitch. Microsoft
explains this better.