I have a tom tom 500, one of the earlier ones, and had the same lack of room problem. I simply bought a bigger card and downloaded new map onto it.
I was buying a complete new map so it was relatively straightforward I don't know how an update would cope though. They are only files so try moving them onto your PC and back onto a blank card.
I must say Tom Tom were exeptionally patient and helpfull in the few problems I did have, related to moving favorites and contacts etc to new card.
Have you tried the Garmin site?