Ok, a couple of things.
Yes, a larger power supply is in order and at least 450 watts. They are very easy to replace. Usually just 4 screws and unplug all the connectors.
You want to verify that the new power supply has all of the necessary connectors for that motherboard. Unless it is really old, that shouldn't be a problem.
The old Video Card is AGP 8x. Make certain the new card is also AGP 8x and you should be good to go. If all you are doing is surfing the internet, you will not notice much of a difference.
If you have less than 512MB of RAM, I would upgrade the RAM to at least 1GB or even 2. That will show you a major improvement.