I didn't make it!
You can spend anything from £10 to £3000 (maybe more, for all I know) on a sound card. 'Normal' cards top out at around £150-200, past that you're into studio recording and that sort of thing. Onboard sound isn't particularly good (as a generalisation), but having said that it's enough for most people.
Sound quality also depends on your speakers...I often think that people will benefit more from new speakers than they will from a new sound card. What speakers do you have?
Performance will probably increase a little, but it's highly unlikely that you'll notice any difference.
It's not something we can really judge...we can't say "Yes! You definately need a new sound card because _________ is happening.". If you're happy with the way it sounds now, then leave it. If you want an improvement, then upgrade your hardware. It's that simple.
Intel does make an HD audio for onboard, which is OK for some speakers. The speaker's capability and design can really matter a lot. Basically onboard is inferior in frequency response and THD. In other words it will cut off sounds above 16Khz and have more distortion, especially if you try to turn up the volume. The midrange and bass will also not be as accurate sounding, especially if you prefer analog sound to digital. I have an M-Audio Revolution 5.1 sound card and paid about $70 for it at Newegg over 2.5 years ago. It has excellent sound quality but limited features. For instance it will not record software sound sources so I have to switch to my Realtek Ac/97 onboard sound when recording Fraps videos of games. The cards I always steer people toward for great value in a high end cards that have lots of features are the Creative X-Fi XtremeGamer Fatal1ty Professional Series and their X-Fi Platinum Fatal1ty Champion Series. They both have the 64MB X-Ram memory for Ultra Audio in games like BF2, and range from $100 for the Pro Series and $130 for the Platinum, depending on the rebates running at the time. If you want to plug in consoles or have a remote to easily navigate through the settings, the Platinum is a great card.
Most aftermarket sound cards will produce a noticeable difference in sound quality coming from my experience. How noticeable depends on, as Archaon said, the speakers you have (Archaon is quite knowledgeable is that area by the way). X-fi series cards are good pieces of hardware, but they generate a lot of heat and the audio processor will only produce a marginal overall increase in system performance; 1-2 fps during gaming average. That may not be worth $100+ to most people. I would suggest upgrading your speakers first (assuming that they need to be), then adding a sound card if you are still not happy with the improvement.