Xi-Fi XtremeMusic PCI

Try it and see, although the quality difference really depends on what kind of speakers you have.

It will be a great improvement over on-board sound quality for an audiophile. But if you are a gamer and want to free up the CPU cycles that the on-board sound chip uses, I believe the card you mention does not have its own processor. That means it will still rob cycles from the CPU. But with SB or IB processors, that's not really a big deal.

Well, it will be an improvement, given how the XM was a higher tier X-fi when it came out. Whether you can hear a difference is dependent on your speaker setup.

CPU usage (onboard vs dedicated) is a non-issue now. There was a time it mattered but not now. I didn't even register 1% CPU usage before I added my sound card when playing CD's in 5.1 surround. CPU's are also rarely a bottleneck so unless your CPU hits 100% usage it's impossible to affect a games frame rate by adding a dedicated audio card.

Audio quality:
As mentioned motherboard audio is adequate for most. You probably need a minimum of $50 stereo, $80 2.1, or $120 5.1 speaker setups before a quality dedicated sound card would sound better.

I compared these two audio solutions with my great M-Audio AV40 stereo speakers:
Realtek 889 vs Auzentech Forte X-Fi (PCIe)

Low quality audio sounds identical to my ears. However, my MUSIC, MOVIES and GAMES often sound far better with the Forte dedicated card. In some of the music I could really hear the separation between instruments, bass was better and I heard noises that the onboard Realtek chip either didn't process or simply washed out.

Thanks to all for the helpful answers. Looks like there is still some life in the old dog. Besides, I paid for those legacy PCI slots, might as well use one of them.   It will be used with a Logitech Z623 2.1 speaker set. My rig will be overkill for almost anything I play anyway, which is mostly MMO's these days.