Well, for starters, you must find out if your motherboard supports and AGP video adapter or a PCI video adapter. This can be discovered by actually looking at the motherboard itself, or by reading the specs on the motherboard make/model on the net.
Once you find out which video adapter type you have to buy, then you have to read the box to make sure that it is compatable with Win98. It will most likely be compatable.
The first thing you should do is read the instructions that came with the card. If instructions are not available.....then you will most likely have to follow these steps:
1) Disable anti-virus software (You should ALWAYS do this first)
2) Install software for video adapter
3) After installing the software and before turning the PC off, goto the Device Manager and DISABLE the old adapter that you will no longer be using.
4) Shut the PC off.
5) Open the PC and install the card. This is a fairly easy task but still, be careful.
6) Once the new card has been installed, plug the monitor connector to the card
7) Turn on the PC. Windows should recognize the card (You may have to point out the drivers from the CD again, but since you installed these first, you should be ok.
8) After the new card has been installed and is working properly, turn the PC off and remove the old video card. If the card is embeded in the motherboard, just simply leave the video adapter DISABLED in the Device Manager.