The best approach to the out of memory problem is to make a full back up using Windows Explorer before you go any further, this way the files removed to make some space can be put back on after the update.
The voices, rastor and zip folder can be removed as long as you have a copy of these folders on the PC BEFORE DELETING THEM, when you have the new map installed you can copy them back to the device:
The "voices" folder contains the non text to speech voice files.
The "zip" folder contains the postcode look up files.
The "rastor" folder contains the European mapping when fully zoomed out.
As an alternate to the above method of deleting files/ folders to make space I would consider purchasing a decent size SD or even SDHC memory card and put the new map on that.
An up to date 720 will support the larger SDHC capacity cards upto the 32GB limit, although the largest cards cost a small fortune at the moment.
If you intend using the built in mp3 player then a decent size memory card can accomodate hours of music and a few maps without any issues 8GB and 16GB sizes are readily available. At 4/8 or 16GB the cost is considerably more bearable. The device will share the built in memory and that available on the storage card, just remember to target Home at the SD card for the new map installation rather than the built in Flash memory - Mike  | 
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