THX&DTS in Sound Card & Speaker, it's worth it?

How much different in term of sound quality if we compare sound cards and speakers with THX, DD or DTS features, to regular sound cards or speakers without those supports? It's really worth it?, as they are more expensive...e.g the Auzentech X-Fi (supports DD Live in upcomming driver) and Logitech Z-5500 speaker.

If the different is minimal i would rather get a mid-range speaker or sound card and be satified with surround sound mode (cheaper Logitech Z-540, etc)

Ugh, this could get really complicated really quickly.

First off, desktop computer speakers (Logitech etc.) are generally pretty crappy -- the weakest link of your sound system by far. IOW it that's what you use for output, it won't really matter what you feed them, as long as it isn't crackly on-board audio. A pair of decent headphones is much better; you can easily tell the difference between a good source and a mediocre one on those.

If you want speakers good enough that the sound card will make a significant difference, I'd say you're looking at mini-monitors at least, something like JBL ControlONE's. Of course, you will need an amplifier for them; similar-quality active speakers are much pricier.

With that out of the way, yeah, the difference an X-Fi makes is real and significant. How much it will matter to you is a different matter -- personally, when playing games, I find that I'm so focused on the screen that the quality of the sound makes little difference to the experience.

Finally, if you're considering gaming, not all games support all of these fancy technologies. IOW, if your game doesn't do THX or DTS, it won't matter that your card does. A good card will still sound better than a bad one, and may be worth the (rather low) price just for that.

(FYI, I have an X-Fi sound card and AKG K240 Studio headphones with it; I have those JBL ControlONE's in my home theater/audio system.)

I'm looking for a versatile computer speaker/sound card for MP3s, watching DVDs and gaming purposes (i did all three quite equally). Logitech speakers certainly can't be compared to hi-fi speakers but they're among the better for computer uses. I wonder what's the different with THX and etc features for watching DVDs. For gaming and Mp3s probably pretty much covered by the 5.1 surround set up and sound card supports...

Mp3s are usually only stereo I think. DTS, THX and DD don´t have much practical value for games. Most only use some strange upmix to get 5.1 - which is still better than no 5.1. I would simply get a carefully chosen Creative Labs sound card for mp3s and games. But only if your speakers are decent. Otherwise stick to onboard sound.
If you really want to use your PC to watch DVDs you should make sure you have at least a DD decoder somewhere. Maybe your DVD-Player software comes with a set of decoders? The simplest and best solution is a bit more expensive though. Buy an external amplfier or receiver with integrated decoders. Nowadays these things have decoders for everything. A cheap one will be sufficient. Double check that you can get the signal from the sound card into the receiver and connect the speakers to receiver.
I think a few speaker sets come with their own decoders.

You have actually chosen a very difficult setup indeed.

As Gorath said, music is a stereo source and while the 5.1 (or higher) satellite setup of a surround system can indeed play in stereo, the sound is usually vastly inferior to a decent 2 speaker stereo setup.

In games you can really benefit from a surround setup to achieve a 3D sound space to go with your 3D visual space but the abbreviation to look for in gaming is neither DTS, DD or THX but rather EAX and that only depends on your sound card, not your speakers.

However, when it comes to watching DVDs then the surround experience is vastly superior in any way to the stereo setup. In fact, I would rather watch a movie with proper surround sound on my 19'' computer monitor than on my 32'' wide screen TV with stereo sound.

If you want to go all out then I suggest you go for a surround setup but then replace the front left and right satellites with proper speakers so you can switch between a stereo setup and a surround setup ... however, this is probably not going to be cheap.