Defragging hard drives

[Easy Mac]

What are the general rules for how often you should defrag hard drives? I ask because I have 2 hard drives, 1 60 gb and 1 200 gb. The 200 gb drive is split into a 140gb partition, a 40 gb partition, and a 20 gb partition. So given the following examples, how often should I defrag the partitions (and where should I place the swap file?

60 gb drive: It has the OS, my programs, my games, my documents, the usual. Since I rarely add anything on this drive anymore, I guess every few weeks is ok?

140 gb partition: It has all of my video files. My computer doubles as a DVR, so I am normally recording anywhere from 0-6 shows a night, then converting the mpegs into avi's at night. I also rip my dvd's and compress them. I shift these files to DVD's after a while (either after I get a whole season, or I put 3-4 movies on a DVD). I'm guessing this partition should be defragmented every night or 2 since its constantly being accessed and has large files.

40 gb partition: This is my music partition. I have about 10 gb right now, and I haven't gotten around to ripping my CD's. Its normally accessed right now just to download podcasts and transfer them to my ipod, so its rarely used. Probably once a month to defrag, maybe?

20 gb partition: Photos. I have no photos, but whenever I get a camera, this is where they will go. So it probably shouldn't be defragged any time soon.

Also, anyone have any good, free defrag software? I'm using diskkeeper light and it seems to work better than windows.

[Cringer]

I remember defragmenting a HD about 5 or 6 years ago. Haven't got around to it since though.....