Try to avoid buying memory cards on ebay unless you have a clear idea about memory cards and the ebay seller. Some cheap chinese cards on ebay could be fakes which either would not work well or would have the capacity inflated by a driver hack.
microSDs come in class 2, 4, 6, 8 and 10. The speed difference between each of these classes is tremendous. However, the price is also different. Class 10 being most expensive and class 2 being cheapest.
You'd have to strike a good balance between speed, price and capacity. I'd recommend a Class 6 with 16GB capacity.
Of course, if you have lots of money to spare, go for a class 10 32GB.