Well the mixing and the song playing is done on computer with AtomixMP3.

But I see now I will need a amplifier.

I believe it's very critical to match the impedances on the sound card and the amplifier... a typical soundcards output impedance is around 8-20 Ohms, and the line input impedance of an average stereo is around 40000 Ohms. Worst case, you might overdrive either side and end up with something burning.

Besides that I have noticed that if you use unshielded cables on a direct connection from PC to a speaker, you get a wierd hum (which I have later found to be a 60Hz hum caused by a ground loop).

A simple solution would be an impedance matching transformer. See if you can find one at an audio equipment store. You can find some specs from http://www.bcae1.com/trnimpmt.htm (funky little calculator thingy at the bottom of the page). I had one custom made for a friend because I couldn't find any over here, but you might be able to find one designed specifically for PCs. I dont know what to do about the ground loop,it just took care of itself when I used a shielded cable.

Hope that helped somehow (most of the information is just from random experience though, might not be correct).

What if we used the same setup we have right now, 2 speakers (450W each) connected to the powered mixer. But instead of having 2 CD players pluged to the mixer, have just the single computer. Would that work?

Because half the time, we get stuck with a broken mixer (it playes the sound, but we cant fade the music for some reason) so I though just having the computer pluged into it so all the mixer has to to is amplify the sound and send it to the speakers, and the mixing part would be taken care of on the computer.