The dpi setting will be put into the image information and used by some applications like Photoshop to calculate a print size.  Dont worry about it in Poser, just work by Pixels.

I only have poser 7 but hopefully its close enough.

1st thing to check is on the render tab of the preview window, there is option for full, quarter, half sized render. Check it says full - see the pic attached

I assume you have the scene saved.

start Poser and load the scene
change to 1 thread only and NOT a seperate process,
start the render and watch your memory statistics from task manager .

If the scene finishes correctly then try it as 1 thread but seperate process
if thats ok then start increasing threads.