Please provide an example, it sounds like PlotName is already rasterized.

Your ImageResolution -> 2000 case works fine on my computer (OS X with 16 GB of RAM) and produces a 185 Meg file.

I have 3GB. I understand then that I arrived to a dead end with Tiff files :(

The EPS files produced with the code in the question looks quite good when converted to PDF and rendered by Adobe Acrobat. They are not blurred in any way, but depending on the settings of the renderer antialiasing may be applied. Here is a screenshot of the first plot in Adobe Acrobat:

As to the journal requirement, I am sure they mean that IF a EPS file contains embedded raster image THEN this image should be 900 - 1200 dpi. If the file does not contain embedded raster images, this requirement is just fulfilled by default.

I recommend you to use EPS format for export since your plots are perfectly compatible with this format. It is surprising that your PDF renderer makes them blurry, what PDF viewer do you use?