I assume this is for file transfer and such. If that is the case, then Apple makes it very easy for you. All you have to do is open the Preferences window and select Enable Disk Mode or something like that in the iPod pane. Then just use the slider to select how much space you want for files vs. music and your done.
yep its as easy as kargner says, i have only 32 mb partitioned off for myself personally, but thats because i wont be carrying anything more than a few word docs back and forth, but you can set it to whatever level you would like