Where did you see that DirectX would run on 98 ? ?
windows 98
COMES with DirectX would be the first clue.
Updating the DX would be pointless in this case, though. the Diamond Monster 3d is DX5 level card, and doesn't even implement Direct3d in hardware, only DirectDraw 1 or 2.
Consider for a moment that DirectX 9.0C has removed DirectDraw entirely and you can sort of understand that there will be no gain.
The only reason to install DX9.0c is to run a DX9.0c game. a DX9.0c hasn't a chance in *censored* of working very well on that machine anyway.
I'd just stick with DirectX 8 on a windows98SE machine.