I understand that Sandisk is generally a faster product than A-Data or Kingston but I'm wondering if there would be much of a speed difference on my 9700 between a 8g Class 4 Sandisk versus an A-Data or Kingston 16gb Class 6. Thanks.

I wouldn't worry too much about it. You're not dealing with a digital camera here. You are going to be mostly reading files and not writing to the memory card.