8db is noticable, but at a sound level of over 100db it's not that great.
I think the drivers for the Audigy 2 ZS suck a little less than the Live drivers.
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You confuse between two terms that sound the same but are completely different. The SNR of your soundcard will determine how low is the noise floor below the maximum level of your card. Therefore, this makes a big difference if you intend to use your card for recording. You should check the output SNR of the card if you want to get a numeric idea of how good is the SNR.
Nevertheless, the soundcard specs are usually not accurate. Given the speakers you use (which might sound good for home uses, but are definately not pro audio monitors) you will hardly notice any difference in SNR (the two cards might sound different due to other differences, but you should be able to notice this).