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300 g = the paper weight not the resolution. What it sounds to me is that you have re-sampled the image. If your planning on selling these posters I suggest you re-think your workflow. I also posted some information in the same thread Mr Tom referred to, please read it and maybe it will clarify things a bit. I'm afraid that  what your doing here is not going to produce the quality you need for this type work.

Also, take a look at this article  http://www.photoshopessentials.com/essentials/print-size/