Maximum (micro) SD card speed class for HTC EVO?
It looks like someone sent HTC an email back in July and got the following response:


  I understand you want to know which speed is the best for your device as far as SD card is concerned. We have tested the devices on a class 4 speed. I could not recommend anything higher than that. The phone may be capable of running a higher-class speed, but we have only tested it on a class 4.


You may also want to email HTC support and see what they officially say.  From I've been reading, people have been using Class 6 just fine.  Not sure about 8 or 10.