The amount of times this has come up should warrant it being a sticky! :)
A 1200x1200 pixel image has the same number of pixels no matter what the dpi (which should really be ppi on screen). The dpi (I'll use that term instead of ppi as that's what everybody is used too) only comes into effect when printing, as a 1200x1200 image printed at 72dpi will be around 16" square, and look like a picture from a newspaper that has been greatly enlarged, whereas the 1200x1200 image printed at 300dpi, will be a 4" square picture that looks very good quality. 
It's not hard really, but if you absolutely need a picture from Poser7 at 300 dpi then just multiply your finished size in inches and multiply by 300, and use those values as your image size. IE, 8" by 10" would be 8x300 = 2400 by 10x300 = 3000. Job done, easy.
If you are rendering large pixel size images in P6, then setting too low a max texture size could impare quality as P6 will downsample the texture to the dimensions you specify. Also, texture filtering can blur textures too. So, for detail in your images, a large pixel dimension is required, not a large dpi setting.
Blimey I went on a bit there :)

Oh, and Happy New Year from England!!