What do you use your system for? Do you have a good sound system to go with it? If you are just using a $20 headset you wont notice a difference, if you have a $200 set of speakers you might, most people cant but there are some people who can tell the difference in sound quality, if you are one of them then it might be worth it.
Well, I suppose it depends on what you do with your computer. For most people I think onboard is fine. If you're going to go that route, I'd avoid the PCI X-Fi cards... no sense in buying one of those since your next motherboard might not have a PCI slot.
Although I do not currently own an X-Fi, there was a time when I did... and I really did feel like it was an upgrade in sound quality. Those are merely OK speakers you have... so I'm not sure how much you'll gain. If you're into gaming, I still think that Creative is the way to go... yes, even with their ULTRA-bad history of crappy drivers. Just make sure you get a real X-Fi and not some card that just happens to carry the X-Fi name. About $80 is the least you'll find a real X-Fi for... read reviews on the EXACT model before you buy. Caveat emptor!
I can feel the difference, but i also have a $100 pair of headphones and $100 set of speakers, looking at reviews of your speakers they dont appear to have the capability to reproduce accurately enough for you to get a benefit from a high end X-FI sound card, most of the reviews complained about distortion at higher volume and static at lower volumes. You might notice a slight increase in quality but it really depends on how your brain is wired.
The latter (Extreme Audio) isn't actually a X-Fi card. It's an Audigy rebranded as X-Fi serie product, enabling some X-Fi features using software.
When it comes to X-Fi based products, Auzentech actually has better products than Creative itself has. Partly due to quality parts used on the soundcard. But Auzentech is more premium priced: 140 euro for Auzentech Forte that offers the best quality when it comes to X-Fi based product that's focused on gaming.
If you are using your computer more for music and movies than for gaming. You could also consider Asus Xonar DX (PCI-e) if you wants bang for bucks audio quality improvements (somewhere around 70 euro).
The Xonar products uses DS3D GX to emulate EAX (1,2,3,4,HD) where as Creative uses Alchemy to re-enable EAX for games (but not every game with EAX is supported actually) in Vista/Windows7. Most games is moving over to OpenAL API which works natively on Vista/Windows7 without the need for a hack like Alchemy. EAX is going to slowly disappear from the gaming landscape. So you decide how important EAX is going to be for you.
In my opinion the biggest factor in sound quality differences with these parts is due to customer expectation, the placebo effect. Drop in some extra dollars and you expect to hear a difference. Caveat: If your current system has a low quality sound chip then it is a different matter.
You should try to find a double blind review of the product you have and the product you are thinking about buying. I have no idea though if those exist.
On the other hand if the add-in sound card offers some usefull functionality over your integrated audio then it might be a worthwhile upgrade.