Using dd to create a disk image?

[oog]

I want to create a disk image of a Windows XP machine (two NTFS partitions) so I can completely wipe the machine and put Ubuntu on it. Would it work for me to use a live cd and run dd to copy the contents of the disk? It would be something like: dd if=/dev/hda | gzip > /mnt/external/image.gz

Is there anything I need to watch out for? Am I better off getting a copy of Acronis True Image or Ghost?