Which 5.1 for my system

When your soundcard (SupremeFX II) is connected using analog connections to the X-540 it will support true 5.1 only if your source is true 5.1.

Some old DVD movies that are not coded with multi-channel only have stereo sound. When you convert the stereo sound to 5.1 through your PC then is it not true 5.1 anymore.

Digital connection is capable of carrying up to 7.1 but it does not mean it’s always carry multi-channel signal; it is only a form of transport to move the signal from A to B. The analog connection requires more wires only if hardware and application is supported by it.

One way to know if you are getting true 5.1 on the X-540 speakers is that you need to turn the matrix switch off from the X-540 and use 6 channels direct on your PC.

I have the X-530 and it's not bad, It doesn't sound like a mini hifi system but it's good enough for most application. On the X540; Logitech added a swicth called matrix, it takes stereo sound a convert it to sorround sound. Check the button on the right on the control unit. HERE

There are better active sounding speakers at the price of the X-540 but it's only 2 speakers and a sub.

The Creative speakers sounds brighter than the Logitech but the sub sounds louder on Logitech speakers than Creative.

No, that was Logitech.

Their speakers are not as crisp as Creative.

The best combo for multimedia to put together would be a sub from Logitech and sat from harman/kardon.