My computer is situated quite far from my stereo system. I can't move either the computer or the amplifier closer to eachother. I have a SoundBlaster Live! PCI card in the computer with a 4 or 5 meters long RCA cable going to the amplifier.
I know that the sound out from the SB Live is quite good with the KX Driver in Windows and with Linux native drivers. But the very long analog cable is very bad for the sound.
My amlifier is a 2 channel, decent sounding, with no digital inputs.

To get a better sound I figured that I could get an external soundcard and place it really close to the amp.
The only external soundcard I found for a decent price is the Sound Blaster Live 24-bit USB.
It is an USB 1.1 device, so the USB cable can be 3 meters long, which would do for me.

I will not do any kind of recording, only listen to mp3s, two channels. What do you think? How much would this improve sound quality?
I do realize that SoundBlaster is not so very popular here. But I am on a tight budget...

Maybe there are some other possibilities/solutions that I have not thought about?

The issue with long unbalanced(RCA) lines is possible noise injection into the signal. If you do not notice an audible noisefloor while playing silence signal, then you don't have any reason to change anything so far as this is concerned.

-Chris

I do not agree with this. There are other factors too. The skin effect for example. What you are saying is that there are no difference between a cable and a cable carrying analog sound, which is not true. But I don't want to make this a discussion of cables and problems concerning different types of cables.

I want the signal going into my amplifier to be as close to the signal that comes out of my soundcard as possible.

The question is if you can hear noise in the signal. If not, then what you have is good enough.

I can't hear much noise. Only if I crank up the volume quite high and have no sound playing.
But if I play an mp3 from my computer, the sound is not as clear and distinct as if I played the same mp3 from my DCD recorder which has a poorer DAC than my SB Live.

The sound is so poor that I want to do something about it. Buying an external soundcard is the cheapest and easiest solution that I have come up with so far.

Avoid Creative Labs like the plague. I'd by an Edirol UA-1EX and a USB extension cable for it. This does true 24bit 96KHz analogue in and out, S/PDIF optical in and out, a headphone amp and has ASIO drivers for bit perfect output. It's slightly more expensive than Creative Labs product but thats because it actually works and does what it says on the tin.

Skin effect is irrelevant at audio frequencies. Skin effect only has substantial effect on the frequency response at much higher frequencies. Beyond the noise consideration are the basic LCR parameters; easily calculable when the source and load impedance are of a known value in addition to knowing the LCR values of the cable used. But even if they are not known, it is not typical for a 25 foot RCA cable(as an example) to have sufficiently large values to cause an audible frequency response deviation in a typical line level circuit.

-Chris