Yes it will be a worthwhile upgrade - depending on what you use your p/c for. If you just do letters, send emails and go on the internet then you wouldn't really notice any different. But with any graphic work and the like or games - yes you will notice a big improvement.
It is quite straight forward to change the graphic cards over. You just delete the reference to the Rage one (for win 98 - start / settings / control panel / system / device manager / display adaptors - then click on the entry for your Rage card and select remove).
Turn your p/c off, open the case and take the old card out and put the new one in. Restart your p/c and then put in the driver CD and away you go.
The only word of caution is that if your p/c is (say) more than 4 years old the power supply might not be high enough to support the new graphic card. Generally you need a 300w one. Again this is not really that much of a problem as it is fairly cheap to buy a new power supply and straight forward to fit.