Backup compression and strategies?

I want to backup some videos/music I have. Currently I have been backing up simply by copying files. I am wondering if backing up entire partitions might achieve better compression.
I am thinking of doing the following.

Partition HDD-1 into 250GB partitions. While HDD-1 is being filled, backup the files to HDD-2. Whenever a partition of HDD-1 gets full, backup the entire partition to HDD-3.

How much space can I expect a backup image of a 250GB partition to take?

 

The amount of compression depends on the file types. If I am imaging a System Partition, it seems like there is around 50% reduction in space needed (i.e., ~12 GB -> ~6 GB). However, if you have a lot of mp3's, video's, etc. there may be very little compression, since formats like mp3, mpeg, etc. are already compressed. Also, you need to reduce the available storage space for the 250 GB partition for formatting and I would further reduce the formatted parition size by 15% since you typically do not want to fill up a partition to "Full".

 

Nice answer.

 

Thank you.

I got the 15% from the Windows XP file defragmenter not wanting to perform a defrag if the free space on a partition is less than 15%.