There are three things that make up what you hear. (four if you include the wiring, but it makes much less of an impact.) The first is the soundcard itself. Over the years this has become less and less of an issue as onboard gets better and better. Usually all you see is better features, like 7.1 instead of just 5.1.
Second is the speakers. Good sound card isn't going to do anything for you if you are running them through cheap $20-$100 speakers. If the speakers can't accurately recreate what the soundcard says to its no better then using onboard. Bose does make good (if not a bit flat) speakers, but are overpriced. Logitech can make good speakers, but as mentioned tends to be heavy on the bass. There are many companies that make speakers, all with good and bad lines. I have found for "best" sound you should run a digital connection to a good home theater setup.
The third thing that will impact your listening enjoyment is your ears. A great soundcard with high end speakers isn't going to do much if your mostly deaf, or can't hear the subtle differences that can be found in music. As a kid I had many ear infections each year, and have difficulty hearing differences in high notes. Spending large amounts of money in audio equipment doesn't make a lot of sense to me as I'll never hear it.
In short, to say your going to buy a soundcard isn't enough info. If your using a junk set of speakers, even if its a 5.1/7.1 setup you'll never hear the differences. More so if your just gaming. Get something that supports EAX or whatever sound setup your game uses and a "good" surround setup. If that means onboard then I'd use that. I'd also upgrade your speakers before the soundcard. A good set of speakers might be enough to breathe new life into your onboard sound.