Resolutions of iPhone3 and iPhone4

So, I've written this entire game with iPhone4 in mind. It also needs to be rendered for iPhone3G and lesser. I know that naming image files with "@2x" on the end of the file is supposed to allow for the program to know which images to load for the required devices. I'm not sure if Unity is recognizing this.

If all of my 2D sprites are designed for iPhone4 resolution do I need to scale down the sprite sheets for iPhone3G? If so, will my program detect this or will I have to write protocols to handle this?

I just need some clear direction on how to best handle this issue.

Thanks