I have a Creative Sound Blaster Audigy 2 ZS, but I believe they renamed and repackaged them a few years ago to fit in with the X-Fi line. For consumer products, Creative Labs easily has the best cards, though honestly any sound card made after the early 1990s will out perform an AC97 chip.
Any "onboard" sound chip is a mooch. They reserve a section of the CPU clock and RAM space for their own personal use. Even then, they don't really keep a cache, meaning that sounds aren't queued up "just in case." Most people with these chips complain of stutters when firing the guns on their aircraft; that's one sound that has to be seeked to and pulled off of the hard drive, loaded to RAM, processed to include directionality for each specific gun fired, fed to the sound chip, then finally fed to the speakers or headset. Most of that time, the game is sitting around waiting for the process to complete and, hence, stops.
A sound card that is fitted in an expansion slot has all of the components it needs on-board. It pulls a sound from the hard drive bus and does everything from that point on locally and on hardware specific to the task. The CPU and RAM that formerly belonged to the sound chip are freed to complete other tasks.
If you do go get yourself a sound card, be sure to disable the AC97 sound chip in BIOS. That way it's sure to give up its stranglehold on the resources and let the card do the work. Sometimes if they're left enabled, they won't let go and the sound card won't function.
