DPI settings should only be an issue when considering the physical size of the print AND the distance at which it will be viewed.  I recently did a piece for a 72"X36" final. I interpolated it 150% to 300dpi.  Then the printer insisted it be in .pdf format not .tif or other common format.  

My latest excursion was to a renowned fine art printer.  He advised working in full size at 300dpi, .tif and never interpolate, even with software designed to do just that.

Some say you are fine resizing to 200% others say never more than 125%.  This last fellow said NEVER.  I kept in mind this last printer does work for museums all over the world.

I will never reach any level of fame but I'm listening to the latter gentleman's advice more than the others.  I'm thinking once I cut off my ear and send it to Rosie and die a tragic death I will be famous. In the time being, I'll will be beefing up the memory in my 'puter and will be having more nap time during renders.

Now about the age thing.  I attribute my little mental lapse to the sugar buzz from too many chocolate chip cookies.  I'm younger than all of you, and prettier, and and and and ... crap I forgot again.