Your applications will not be the issue. The fastest drive should always be true audio and sample related data. Midi data is a non-issue.

How do you intend to configure the 3 drives, because it will make a difference.

Your main apps drive PATA should be the boot drive, Primary Master. The main Audio data drive should probably be the Secondary Master. If video is to be the heavy use, I'd think I'd make it the SATA device 1. You can only have 2 SATA drives on that board as I recall and that would be the 3rd channel, SATA??

I can check this tomorrow, but not where I'm at now.

i see what your saying but a SATA drive for video is out of the question. the drives are too small and expensive for the amount of data that i will be putting on them.
this is the config i thought would be best?

SATA WD Raptor (boot drive, os and apps)
PATA Primary Master Audio Drive
PATA Primary Slave DVD RW
PATA Secondary Master Video Drive
PATA Secondary slave ?

The best config is a jroberts said, but that doesn't solve your space dilemma. I'd suggest using the raptor for playback of video, and using audio drive for storage of older video. Keep the raptor clean and unfragmented, and transfer stuff to it as you need.

Having a faster boot/apps disk will make a small difference, but basically only when starting the OS or starting the app, because it will load to RAM a little faster. But the OS and the apps will only run faster if they make a lot of calls to their code on the hard drive. But nowadays, an OS or app uses RAM to hold the code it needs to execute 80% of its functions.