I would definitely get another 2GB stick (preferably one identical to your current stick). While it is true with a 32-bit OS, you will not see the full 4GB, it is still being used by the OS for hardware mapping. If you just install a new 1GB stick, you will end up with less useable memory. This is particularly important if you are using integrated graphics.
Enabling PAE often causes more problems with compatibility issues than its worth. I don't recommend it.
A better idea is to bite the bullet and get a new computer with W10. If your current system is running Vista, it surely is pretty old. That means even finding compatible RAM may be a challenge and expensive. You likely will have to go used and that always comes with risks. So even a budget computer today will like run circles around that old machine.