Garden Style typically means that it is 2 or 3 story buildings as opposed to a high rise, mid rise or townhouse style. The different qualities of construction can make a difference as to noise level also. Class A construction would mean that there is cement floors between levels. Again typically most garden style building were build of frame construction and would have wood floors between levels, carpeting would help soften the sound. Also that is why the top level units command more in price.
The one thiing to be aware of on any condominium is if they have a healthy reserve and if there are any upcoming capital improvements that will require a special assessment.
Re garden style condos - this probably has more to do with how common this style complex is in the area where you're looking. Truthfully, it's not a term that I've ever heard used outside of the MLS building-style field. My understanding is that it refers to condo buildings of 2 or 2 stories.
Buildings vary greatly in terms of noise transference. I think that many of the larger, more modern buildings with concrete between the levels tend to be quite good in terms of minimizing noise transference. Wood framed buildings can go either way - in some it seems that your neighbor upstairs is in your home, in others there's minimal sound from your neighbors. David gave good advice about trying to visit the condo at times when the neighbors are home.