I use a
creative usb soundblaster 5.1 . It cost £40 and comes with all the leeds you need to attach it to your mixer along with some recording software. It will record at 24bit 96Khz which is better than CD quality.
As somone else has allready mentioned - - dont have your mixer in the red on the output. Idealy you want the levels to go up to 0db but not over. And keep an eye on it. If you get over excited and start turning up the levels then you will start to get "clipping" which will make your recording sound distorted