If you check out the official Sprint Evo Web page and click on View Full specs you should see that it will support Micro SD cards up to 32gb which is a HC card or High Capacity card. This would also allow it to be backwards compatible with any micro SD card out as far as I am aware.
http://now.sprint.com/evo/?id9=SEM_Google_P_Sprint_HTC
right now Sandisk offers 32gb cards for about 200 bucks. I am sure they are cheaper but due to the fact they are still new on the market you will pay a premium for them. Stick with the 16gb which can be found for 60 to 80 bucks and then upgrade 6 months from now and probably get it at half the price.