Almost any hard disk will have more bandwidth capability than your 100mbit Ethernet connection to the machine, so you don't have to get too concerned about the precise drive specs.
You might consider the fact that machines that old are likely to have a BIOS that will only support a 32gig drive directly. I'd check and see if there's a BIOS upgrade for the machine in question to support at least 127gig drives.
I'd forget the RAID and just do some sort of automated backup to the second drive, much greater data security.
If you want real security, don't use W98 for your server, use 2K or XP.