As stated earlier, unless you have a nice set of speakers, or you want to connect a 5-channel or better setup and use a microphone, onboard should be fine.
Personally, I'm an audiophile. I have a set of klipsch Promedia Ultra 5.1 speakers connected to an external sound card (I didn't want internal because it can produce low levels of interference). If you had a setup like this, you would definitely notice a difference between onboard sound and using my card. The PCI card you have is a little old itself, though, so even if you did have a nice set of speakers, you would probably want to buy one of the newer cards instead of using either of those. If not though, it isn't worth the hassle to mess with installing a PCI card for a performance increase that you probably wouldn't ever notice.