If you only use your pc for gaming, probably nothing.
As soon as you start to do other stuff like recording, you'll notice a huge difference between an on-board and an aftermarket soundcard. Even if you're just listening to music there is a significant difference already.
I had some Sounblaster card from creative that was almost 10 years old. It still produced better sound than the onboard soundcard of my about 3 year old asus motherboard. I replaced it with an X-Fi Titanium about a year ago, which again, was quite a big difference.
I guess it's not the best soundcard ever, but especially for recording, it's providing far far better results than any onboard soundcard i've ever tried (they are almost useless to be honest).
I can recommend the X-Fi Titanium (when it comes to music and recording), but can't say a lot about it regarding games. I rarely play any games on the pc anymore. Iracing worked perfectly, other than that, i have no gaming experience with the card.
You wont know whats wrong with onboard 'till you try something different :P
As an experiment I put a cheap-o SB Live! in my old PC (with an A8N-SLi board) and straight away I could hear the difference even with a £2 sound card. Later on I got an X-Fi XtremeMusic and the difference was bigger again...
@Tomba - why the Titanium? Unless you need the extra IO then the Music gives the same SQ as the Music (and of course gives you more money to mod it if you so chose).
You could also have a look at cards like the Audiophile 2496 from M-Audio, which is supposed to be quite good and within your price range probably.
There's quite a few people on recording forums that more or less clearly steer away from anything that says "Creative" or "SoundBlaster".
I've read somewhere that if you are interested in an M-Audio Revolution 7.1, and you don't really plan on using a 7.1 setup, the Revolution 5.1 would be the better choice, since it is supposed to have better DSP (digital signal processing).
Ohh and by the way... i got it mixed up. I don't have the titanium. I only got the Xtreme Music.