I think you will find the Zumo has a limited amout of memory. I have found this out when I was uploading my POI's. I even went as far as to remove the foriegn jibberish, and only kept the english talk. I suppose if you where to reduce the pixel resulotion in PS you could make up for some space, just depends on how much detail and how many. I want to say the human eye can detect resolution reduction at around 150DPI.
Check out ebay for sandisk deals, I have gotten some sweet package deals on the buy it now side of the fence.
Typically, I keep my routes, POI's, and low res images relating to a known POI on my Zumo.
All music files are stored on disc.
"Pain is weakness leaving the body"
"Nothings possible until its done!"
"If ya can't afford the fine, take the lead!"