You need to connect the output of your 250 to a external sound card that you would have hooked up to your computer. Make sure you do not exceed 0db while recording and after the set (on your computer) you can boost the signal. You do not want distortion. A lot of guys use Audacity to record sets and it's free. Here's a link http://audacity.sourceforge.net/
S/N ratio stands for Signal to Noise ratio. Refers to the level of your signal to the level of the background noise. If your just recording your mixes, I wouldn't worry so much as I (IMO) don't think you would notice the difference.
Here's a more detailed explanation on SNR.
http://en.wikipedia.org/wiki/Signal-to-noise_ratio
Good luck