If you really want to partition your HD, go with Partition Magic 8. It allows me to resize partitions on the fly, make partitions on the fly, and do pretty much anything you want related to partitions ON THE FLY. What this means is in Win XP say, I can create a FAT32 partition at the beginning of my drive, reboot to this partition, install win 98, and then have the included Boot Magic utitlity allow me to select what OS I want at startup. Simple. I can then size them differently, moving around gigs of information or blank space without loosing anything. I've been fooling around with the sizes on my 80GB HD.
The other guys are right about the sizes. Go with atleast a gig for '98, and leave XP have 2 or 3 gig. Give the rest to your "storage bin". I just the left the /program files for the specific OS's alone so they wouldn't cause problems. Partition magic automatically hides the primary partitions from one another, but you can choose to unhide if you like, although this can cause serious problems with '98. Win XP seeing the '98 partition is fine. Good luck!
