I'm not sure how fast the 10G and 40G are, but I guess you don't care since you have a 160G SATA. I don't think you'd want to go any lower than 10G for XP. 5G is plenty for 98, provided it can access the 160G (in other words the 160G is FAT32). I'd think you'd want at least 10G for Linux if you are going to put significant things besides the Linux OS on that partition. If you can repartition the 160G and add another 5G linux partition, 5G for the Linux OS is plenty.
Personally, I have a commercial boot manager that makes multiple boots fairly straight forward. I understand there are freeware boot managers that do the job. Linux has a few. How you do more than two boots with the linux loaders (two is what the Linux setup programs will do for you), I don't know.
If you do things in the right order, you don't actually need a further stand-alone boot loader.
I hope I have this straight, because I have never done it this way.
Install 98 first. I suppose you'd want it on a partition other than C:, so you will have XP on C: (Of course XP willl reside on any partition you like too, if you prefer.) Then install XP. XP has its own boot manager built in, and will automatically set it up so you can boot 98 or XP. Just have the two OSes on separate partitions. Actually both XP and 98 have a couple of startup files on the C drive, but neither have to have their "system" files there. As I say, the XP install recognizes this and takes care of it. You can add as many more version of XP as you like. I have three. The XP loader lets me select any of those and 98.
Then install Linux. Linux has its own boot manager, and I believe it will set it up so you can choose XP or Linux. To get to 98, choose XP. That will get you to XPs boot manager. Then XPs boot manager will allow you to select 98.
The upshot of this is that it is easiest when you are doing all fresh installs.
You can accomplish the same thing while installing in the wrong order, but it gets to be quite a bit more tricky and detailed and more work and more error prone. Just to avoid that kind of hassle, that's basically why I stick with a stand-alone boot manager. I am also free to dump any OS at any time (which I do!)
If the SATA controller is a secondary controller, not built into the chipset, you have the problem that it will need a driver to install any of the OSes on HDs connected to it (I believe), and each OS has its own method for getting that driver while installing. SATA in itself is supposed to be transparent, it doesn't need a driver. But secondary controllers on another chip do need a driver.
If the OS is not being installed on a HD on a secondary controller, you can add the driver for it later, and then get access to the drives attached to it. I'd still have the installer load the driver if possible.
You can install 98 after you install XP, but the 98 installer will wipe out the boot sector that XP needs, and put its own there. Then you can't get to XP. Although there may be other ways to fix that, reinstalling XP over itself is easy, and the least error prone, although time consuming. That type of installation will retain your settings. The installer will look for versions of XP and ask if you want to repair one, and that is what you choose.
(Don't choose repair console. That is a command line version of XP that you can use to repair some things, if you know what to do.)
A free boot manager. Looks good!