Thank you for taking the time to contact HTC America Technical Support via E-Mail with your inquiry about what class of SD card to use in your Sprint HTC Hero. My name is A.J. and I will be happy to assist you today.
If you are looking to purchase an SD card that the device will read at its full native speed you will need to purchase a class 2. The device will read a class 4 but not at its native speeds.
Frank, if you have any future questions about your device please feel free to visit HTC - Touch Phone, PDA Phone, Smartphone, Mobile Computer and send us another e-mail. You may also enjoy HTC Smartphone Wiki - HTC, which has a plethora of information on HTC devices, including the Sprint HTC Hero! I would also like to invite you to participate in a customer satisfaction survey located at HTC Online Survey I hope you have a great day!
HTC America Technical Support
Thought I would share from xda-developer forum.. response from HTC
Some have mentioned though that they have seen speeds faster then Class 2 ??
mine is a class 6 and it works fine, when i changed out the 2 mb to the new one, I just selected all and copied everything over, make sure you have hidden folders viewable though, there are a couple you wont get that certain apps put on it