Can anyone comment on my situation?  I have been using an HP 8750 for a long time now and previously I always used 300ppi for prints.  After reading this thread, and some other similar info from various photography forums, I've been printing with 600ppi.  I use the printer's "Best" quality setting and usually print with Qimage.  I honestly am not really sure if I'm seeing a difference between 300 and 600ppi.

Should I continue with 600ppi?  Should I be printing at "Maximum Quality" (interpolated 4800x1200) setting instead of "Best" quality?  I print from 16 bit Tif files in Adobe RGB, and use the canned HP paper profiles with HP Premium Plus paper.