The US and Canada map set uses up almost 900MB alone(for the 675 map). The rest is taken up by voice, POI, and application files.
You can buy a 2GB SD card and copy the contents of your 1GB card over to it, which will give you the extra space you want/need. The real key is making sure the card you buy will work with your device if you go with a 4GB card.
I just did a brief Google search on this, and make sure the new card is formatted properly. Most SD cards come pre-formatted with a FAT filesystem, which will not work well with a 4GB card, and will never be able to work on a card that is larger than 4GB.
Now, I have a Palm Treo 680 as my device, so I know what types of cards it can handle, so I just buy a new card, and format the card on my PC(with a card reader) using the FAT32 file system, and the larger cards work fine.
The reason there can be a problem with larger cards is the support for FAT32 may not be there, or the device you use does not know how to format using the FAT32 filesystem. The reason for this is that when a filesystem is designed, the design specifies how many bits are supported for the filesystem, as well as the number of bits used to record the file sizes. FAT/FAT16 support up to 4GB total filesystem size, which is a 32 bit number for the total size. FAT32 supports a larger size, but it isn't THAT big, it goes up to around 40 gigs or so, maybe 60 gigs, which is a huge improvement.
FAT32 has a limit of 32 bits for the size of an individual file though, which is why you can't copy files larger than 4GB to/from a FAT32 file system. This is where NTFS comes in(supported by Windows NT, 2000, XP, and Vista).
So, for now, all devices support a FAT filesystem, but not all will work with FAT32 formatted cards. That is the question on the Tomtom devices that I can't answer. Anyway, here is the info on SD cards that should help you:
Here is the link to the discussion I was reading:
http://discussion.treocentral.com/showthread.php?p=1202815
And the information you need from that thread:
Originally Posted by Barnel View Post
Very important is the cluster size of your card. You will get the best results with the following cluster sizes. Use the DOS box for formatting your card. I`m using a Transcend 4 GB / 150x with best results (16K), even with big Western Europe map.
Less than 4 GB (FAT16):
2 GB SD (SanDisk) FAT16 mit 32K: format X: /FS:FAT /A:32K
4 GB and more (FAT32):
4 GB SD or SDHC (Transcend) FAT32 mit 16K: format X: /FS:FAT32 /A:16K
"X" is the drive letter.
