I'll look through my pile of CDs when I get a chance, I *might* have a 98 install CD with a key stuck in it. If I find it's all yours. (No guarantees, I've chucked a lot of stuff over the last few years.)
Easy Peasy. Just partition the drive when you install 98 to not use all of it. (Somewhere between 2 to 8 GB or so is more than enough for Windows 98.) Then run a linux installer, it should detect the Windows installation and offer to add it to the GRUB boot menu when you set that up. (Or, heck, even if you do give the whole drive to Windows initially most Linux installers are capable of resizing the partition to make room.) It gets complicated to do Linux first because even if you partition your drive correctly to leave space for the Windows install to go in Windows will want to blow away the MBR boot record on the hard disk and replace it with its own, thereby wiping out GRUB. To fix that you'd have to boot a Linux rescue CD, re-run the GRUB setup, deal with that screaming thing again, etc.
Note of course I mean *real* dual boot, not a Wubi install. Don't go there.
If you install an *NT* based system (not 98) then it's technically *possible* to install GRUB to the Linux /boot partition (which has to be a "primary" partition) and fiddle around with the NT boot manager to boot it that way instead of letting GRUB run the show, but it's substantial additional pain for zero gain. It's almost always best to just let the Microsoft OS think it's alone at install time and add Linux after the fact so Windows won't have to worry its pretty little head about it.