If you are only sending the file to be viewed onsceen, there's no need to worry about the DPI. DPI is only used when converting pixel size to physical print size. If you want the picture to be viewable onsceen, then just reduce the pixel size to fit. DPI isn't used by onsceen image viewing programs unless maybe in print preview.

You can also select certain areas of the z-buffer/distance masks with Photoshop's "color range". This way you can modify the "camera's" place of focus. For example if you do color range with 50% black (grey) and then invert the selection you can blur only the foreground and the background. The middle area of the image is then maintained in focus.

If you are trying to make this on a picture made in a 3D Program, you could see if your program supports saving the z-buffer. z-buffer has the depth information of the image and can be used as a mask when using gaussian blur as advised by lundqvist on the previous reply. If you are editing a photograph you have to make the mask by hand as photos don't have depth information.

I have also experienced these errors... Doesn't seem to be related to memory running out though. I suspect it's a bug in version 6. This error has trashed my work couple of times on different machines. The other one has 512 of memory and the second has 256...

I suppose you are using Imageready to do the conversion. Try putting a color in the matte box. The matte option can be changed from the same place that has all the quality controls (convert options or something like that). Hope this helps you...