Hi Phil, i am much the same with GPS stuff, thats why i bought the 400, and not the 550.
The basic stuff like, the ability to see where you are, or find a street / road is all i really need, and having the ability for listening to MP3's is a bonus.
Where i find it gets more technical is when you try to plan journeys on the computer using mapsource and loading it onto the Zumo.
The same can also be said for music, in particular the MP3 tracks, again you have to figure out if you need to "convert" your music to the MP3 format, then sort out play lists to load onto the Zumo. I found Windows Media Player to complicated to do this, and tried a program Called Media Monkey, recommended over on the Zumo Forums. After a while i figured out how to use it and now have about 100 tracks, all in play lists from different artists.
You will probably need a 2 Gig SD Memory card, as the 400 doesn't have a lot of spare memory. My Memory.com supplied me with a 2Gig card for £6.99 post free from Jersey.
Theres a lot of good advise around here, and i can recommend the Zumo forums
http://www.zumoforums.com/ ( you have to register to get full access)as a great resource for learning, lots of tutorials and pics / screen shots on all topics.