Up to 32 is supported.

 

The speed of the card doesn't matter much in stock configuration. The included 8GB is a class 2 (slowest). If you root and use a rom with the latest sd card fix, a faster class 6 or above card can make a difference in the overall speed of the phone. If you plan to stay stock, go cheap. If you plan to root and flash, pay a bit more for a higher class.

 

First of all class rating does not mean the R/W IOPs are bound to that. All it means is the minimum rating has to meet that requirement. For example SanDisk Class 2 can actually out preforme Kingston Class 4 due to it being superior quality, now that being said.

As most device is under SD2.0 and the device usage is post class 4-6. I myself did alot of benchmarks on mSD if you ask me go buy Patriot LX 32GB Class 10 its only $99 as its the best deal compared to reputable dealers like SanDisk 32GB Class 2 and Kingston 32GB Class 4.