Is all your memory being reconised? Try reseating the memory cards! Are the card in the right order in the slots/banks? Check bios to see if any memory is being set aside for graphics card or something else.
Was there any problems installing the win98 operating system, might be worth checking the registry to see if there is any corrupt files laying around.
Another querry, I'm not 100% sure weather win98 will support a 120gig drive, it might be worth concidering partitioning the hard drive into smaller partitions, check m$ support site should give you more info concerning this.
Hopefully the above might give you some more ideas.