The Z3200 User Guide describes two possible rendering resolutions, either 300 or 600 PPI. I normally send an image to the printer at or near a resolution of 240 PPI. I am happy with the results of the printer driver's rendering of that image resolution. What are your thoughts about up sampling in PS (if required) to match the minimum rendering resolution of 300PPI? 600PPI? To my eye it doesn't seem required
You're talking two unrelated numbers. The image resolution of 240 ppi that you're sending to the printer from Photoshop is just the 'number of dots' in the image, and anything above 240 (or even 180) will print pretty well. Supposedly the nominal incoming image resolution the HP printers are set up to handle is 300 ppi.
The "rendering resolution" of 300 or 600 ppi is a completely separate issue. This has to do (I believe) with the dithering algorithms applied by the printer, and they are actually rather arbitrary numbers. My personal experience with the HP Z3100 is that the higher resolution setting of 600 ppi avoids the banding I'll otherwise occasionally see on long panoramic prints with glossier papers. I can't see much difference on rag/matte papers.
Hope that helps.